I’ve just gone through the same thing.
The person who I consider my best friend who I’ve been friends with for 25 years has gradually, but very obviously dropped me. It wouldn’t feel as bad except she knows I’ve been through the same thing with family and I’ve been so ill too.
Started with her being weird and overly-jokey /defensive in texts. Then she changed to emailing me instead but not even using my new/usual email address, but using the old one I’d told her I barely accessed.
I then got a lovely birthday card saying all the right things. I replied to her email again and she completely ignored it. Again. I know it won’t be completely the same for you, but I figure one of the following applies:
-Her husband is isolating her or doesn’t like me.
-My family (who she knows) got to her
-They also have a shared religion, and the prejudice/brainwashing over my having left it might have overridden any goodness or humanity
-We’ve both changed over the years and she wants to move on. Or maybe I’ve changed; I just started a new career.
-I’ve said something inadvertently to trigger something I was unaware of or there’s something she never liked.
Thing is, most of this just doesn’t involve me doing anything wrong. I also (luckily) know other people who like me. I’ve experienced this before and it’s usually because the other person has developed some kind of new snobbishness based on a new life (have had friends ditch me when they get married) or they have a massive prejudice against me which doesn’t say much for them (sil can’t stand me because I’m different and I refuse to fit the mold).
Main point is though, it’s nearly always their issue. If not, you’d probably know that you’d done something upsetting. This is so hurtful. But if someone is capable of this, then…?