Oh MP Am so, so, sorry you are having such a hard time of it. I bet it's especially hard as she was doing so well before all this. Having said that, I think the positive thing is that she was so good and she won't have suddenly forgotten how to put herself to sleep. Therefore there must be something specific causing this and once you've cracked that, she'll go back to her lovely, happy self In the meantime, is there anyone (family/friends)who can help you during the day so you can catch up on some sleep?
This is quite a long post as I've been trying to rack my brains and think of anything that might help you. I think the problem is either teething or hunger or maybe a combination of both. If I were you, I'd tackle the food first as there's nothing you can do about teeth and teeth also seems to come in fits and starts (IME anyway)
My DS has an 8oz bottle when he wakes, anywhere between 5.00-6.30am at the moment then at about 8.00am he has porridge which he loves. Once we've done the school run, he has a bit of my toast (although he ends up wearing most of it rather than eating it). At about 11.30am he has lunch which is usually 3-4 cubes of vegetable puree mixed with some of his milk and a bit of baby rice if it's watery. This week, I'm going to start giving him some protein (chicken, red meat etc) at lunchtime as their iron stores start to run out at six months and can't be replaced by milk alone. This is what I think is causing my DS to wake so early at the moment. After his lunch he has a bottle (usually about 4-5 oz) although I'm trying to phase this out and am slowly increasing the solids he takes.
He then has a bottle at about 3.00pm before we do the school run and usually has about 6-8oz. He has tea at about 5.00pm which is 2-3 cubes of fruit puree mixed with baby rice and then has a final 8oz bottle at 6.30pm before bedtime.
My DS seems to be back to settling well and last night I didn't hear a murmur from him from 6.50pm when he went to sleep until just after 12.00am. I settled him in about 5 minutes by putting his blankie over his face. This is a Kaloo teddy which has a square piece of fleece for it's body and DS can suck on this as comfort and also likes to put it over his face to go to sleep. I've been putting this in his cot with him at every sleep for ages now so maybe something like this might help your DD. I ofetn hear him in the night sucking on his teddy before going back to sleep himself.
He then woke again at 5.00am, I fed him but he didn't go back to sleep until 6.30am. This early morning waking I think is now down to protein as I said already so that's next on my list of things to tackle.
I think once you introduce food, you may see some improvements. You also said you were BF'ing. Do you give any bottles and if not, do you think this might help. Maybe some formula might fill her tummy a bit more. I know lots of friends who've introduced bottles for the afternoon and evening feeds at about six months.
Sorry, lots of rambling but I hope some of it might be of use. Chin up, I know it's hard (as I've been there) but it WILL get better