I deleted someone off my FB when I was going through IVF and miscarriages, because she'd been really insensitive online. I found it upsetting and it made me very quick to block other people when they announced their pregnancies. It's possible that someone else has put up something that made her feel awful and she's reacted by cutting off everyone that has the potential to make her feel worse about not being pg - not saying that that's right, but that you shouldn't take it personally.
Tethersend's right - she'll be more upset than you are, and it's a self-defence thing. You can be friends IRL without being on Facebook together, and I'm sure she is happy for you, she just needs some space right now. E-mailing her to to add you back when she feels better is a good idea.
And I also suspect it's a real effort for her to comment on other people's baby pics, and in her mind coping with pregnancy statuses is a bridge too far at the moment?
It might also be that she doesn't want you to feel guilty about posting stuff and that's a factor in deleting you? I was really distant with pg friends partly because it hurt me to see other people's pregnancies, but I also thought they would feel awkward around me too.
Don't feel bad, just enjoy your pregnancy but cut her some slack, and it will come right in the end.