Why would a co-worker suddenly be not buddy buddy anymore and be polite but not the same?

i have a coworker friend that i've been friends with for a few years. i noticed a marked diff the last few months where she suddenly stopped iming and doesn't initiate any contact (emails, ims) anymore. we used to share alot (work stuff and gripes) and now it's odd because she used to im me everyday and be all friendly and now she never im's me and always me initiating which i don't do too much because obviously i don't want to bother her if she doesn't want to be friendly anymore. we don't hang outside the office too much it's more of an office friendship but just wondering if anyone else has experienced this. i did ask her awhile back if everything was ok and she said oh sure and that's she's been busy but i can tell it's more than that but whatever it is she's not going to fess up to. frustrating!