Sometimes you just have to let things go though.
Would it really help to talk about how you feel?
It wasn't her greatest friendship moment, but at seven months pregnant she probably has a lot on her mind, from her health to her finances to how near to the end of the pregnancy she is and all the things she still has to do to get ready for that.
Those last couple of months can be really hard. I remember that by seven months I had no clothes that fit, no shoes that fit (I mean, shoes! I was too fat for my shoes!), everything I ate made me sick or gave me acid indigestion, I had mood swings so extreme that DH says I gave up speaking and just took to growling at everybody, I couldn't sleep, I couldn't sit down without bits hurting or stand up without different bits hurting, I was starting to feel very scared about how this giant baby was going to make it's way out of me, I could hardly walk, I was enormous, so big that a woman expecting twins wanted to know if I was having triplets, and a million and one other things were on my mind. If your friend has half of that going on, she's not going to have anybody's birthday but her baby's on her mind right now.
She remembered during a text conversation and sounded genuinely bothered that she had forgotten until that point. She may well send you a belated card or gift on Monday when the shops and post office are open. It's possible she had something ready to send but forgot to post it last week.
But please try to focus on the positives of your day. You've said that it was a lovely day, you've said other friends and family made the effort. If you are feeling sorry for yourself over the one person who forget rather than feeling lucky to have several people who did remember, you're going to hurt their feelings as well.