I'm also not a psychologist, but is it possible she just then realized that she was following a client? I assume you don't use your actual name on Instagram, so maybe she didn't realize it was you at first? Or was waiting to see if you continued seeing her as a therapist before unfollowing? Like if you just saw her once or twice, maybe she'd have continued following. I imagine it's for professional/boundary reasons, rather than her not liking you.

I agree that you should ask her.