How strange. I don't know - I had a friend who just suddenly lost interest after we'd been really close. We were really good friends for a few yrs and though me having ds1. When I was pg with ds2 she was pg with her dd1. We were still close all through our pregnancies but after the babies were born she would just never be around to meet up, always busy (but I knew she didn't really have any other mummy friends). I kept emailing now and then but her replies were always really short and abrupt. I got the distinct message she wanted me to leave her alone so that's what I've done and that was 2.5 yrs ago
It's really, really odd - it doesn't affect my everyday life - I've got tons of friends and a great network of them with children, but I do every now and then wonder what on earth happened there. It's like I need 'closure'! I can't remember doing anything to upset her and she wasn't backwards in coming forwards so she'd have told me.
If i were you, if I happened to see this friend I'd bring it up - just take a deep breath and tell her you are upset by the impression you are getting that she doesn't want to know you, have you done something to upset her. If you don't see her (like I didn't with my friend) just leave it and put it down to a bad experience