YANBU.
I had a "friend" like that - dumped us for a bloke when we were young teens (many, many years before FB... because I'm old 
), and then, when she was mysteriously single once more, came crawling back in the hope that she could pick us up where she'd dropped us.
My group tolerated it for a while, because she was considered "the fun one" - y'know, the giggly sort who always seems to be revered by teenagers/young adults. I tolerated it for a while longer, because she and I had known one another since our first day at school. However, because I'd known her since we were both 5, and not merely since we were 12/13, I knew her a little better than them. I knew that - when we were in Infants and Junior school, she was prone to dumping one group of friends for another if she thought they were "better". For some reason, though, she and I'd remained friends through all of that. The clique-hopping. To this day, I'm not entirely sure why. I was a bookish and very shy type of child. Anyway, I tolerated her behaviour/attitude for a while longer. When we were 15, though, and she "fell in love" with an older bloke (20, if I remember rightly), she dumped me, too. Which was fine. Whilst our group of friends had ceased to be her group of friends... they were still mine. We all rolled our eyes and made bets as to how long it would last and what we'd say to her when she came crawling back... Except she didn't. And that was also fine. I'd see her around town sometimes (we were at different schools by this point), always giggling in the middle of a new group of friends, or with different blokes - and she looked happy. Seemed happy. Which was great. I put it down to the fact we were at different schools and had different interests.
Five years later, when I was pregnant with my DD, I bumped into her on my way to work. "We must meet up!" and "I'm so sorry - but wow! You're pregnant!!" conversation happened and we exchanged contact details. For the rest of my pregnancy/early foray into motherhood, she was always around. I remember her arriving a few days before I gave birth, with expensive baby sleep suits, and a frame to put my DD's first photograph in. And I thought that maybe she'd grown up a bit, grown out of treating us all like the second choice. I asked her to be my DD's godmother.
When my DD was 2 and her son was 1... she dropped me again. An early "ghosting", actually. She simply stopped answering her 'phone to me, would ignore us if she saw us in the street... would ignore my tiny DD, who didn't understand why... and pulled her little son away from us if he tried to toddle up to us. It was awkward, because she lived in the next street to us at the time, but not for long because she moved. My group of friends all acknowledged how shitty her behaviour was (who ignores a toddler trying to say "hello" to a woman she'd always called 'Aunty'?!). I - like so many modern ghosting survivors - didn't know whether I'd said/done something to upset/offend her...
But I got over it.
When my son was a baby, and we were waiting outside the school gates for my (considerably older) DD... I think it was about 7 years later... she approached me. Her son was in the year below my DD at the same Junior school I (and my former "friend") had attended, so I knew there was a possibility that I might bump into her. That my DD, who still talked about her 'Aunty' X every now and then, might see her and, potentially, be upset by/about it. When she approached me, she must have known how shitty her past behaviour had been, because she was very hesitant. I remember it clearly. She congratulated me on my new baby, and made awkward small talk for a minute or two, with me stonily doing my best to not give more than a one word response to her questions. And then... out it came. Her excuse for having "ghosted" me all those years earlier - she'd been depressed, she said. In an abusive relationship with her son's father, she said, and I wouldn't understand (except I would have, because my relationship with DD's biological father was incredibly abusive - which she knew all about, because we'd discussed it several times prior to my DD's birth!). And that was why she went NC with myself and my small daughter. But she wanted to be friends again. Would I forgive her?
Stupidly, I did. I put it down to sleep deprivation clouding my better judgement, to be quite frank. My group of friends collectively pulled that face you get when you've announced a stupid decision and they know you'll regret it... but they won't be the ones to tell you, because they love you, and they want you to be happy. That face. The 
face. But she and I'd meet up every now and then for a coffee, and my DD was thrilled to have 'Aunty' X back in her life again because she'd taken her son under her wing at school and was looking after him (he has mild learning difficulties and a physical disability - so I actually thought that might have been why X had been depressed enough to cut us loose the first time). It lasted for a couple of years. She'd had her first daughter - by a new bloke - within the first year, and her second daughter 10 or 11 months after her first, because she was desperate to hold onto the bloke. I think it was my turn to pull the face when she told me this, whilst pregnant with her second daughter and as I hauled her first daughter around in her buggy because X was too afraid of miscarrying...
She texted me to say that she'd had her second daughter - and then immediately dropped off the radar. I bumped into her mum whilst I was in town one day and ascertained that she and the children were all okay... and that was that. I've not spoken to her since - and that was knocking on for 10 years ago, now. Her DS went to the same senior school as my DD, a year below, and is apparently "a nice enough kid"... but my DD wouldn't be seen around him, because he was younger. She came home one day and said that she'd seen 'X' rushing into the school "in her pyjamas, with the girls hanging off her", but my DD didn't say if she'd tried to say "hello" to her and been ignored, or if she'd ignored X totally. I didn't ask.
I do know that she married the new bloke... and they were divorced within a few years, with him gaining custody of the two girls. Her DS is friends with the son of a friend, who doesn't know that I used to know X and who likes to gossip. I felt a little pang of sympathy for her, I'll admit, but knew then that if X ever approached me again, it'd be me ignoring her. Fool me once, and all that. More importantly, I will never forgive her for the way she picked my kids up and then dropped them again as and when it suited her. My DD adored her - because she was fun to be around, as far as DD was concerned, whilst DS was more reticent. But he grew very fond of her elder daughter in the time X was back in our lives, and he was maybe 3 years old when suddenly they were gone again. Like DD, he didn't understand why we weren't meeting up with X and her first daughter for coffee, like we had done. He cried the first few times he asked to see them and I told him "no". It's that which I won't forgive.
So no, OP, you're not being unreasonable in the slightest to want to protect yourself from being used by this fairweather friend again.