Quick rant before I go off

I've been feeling a strange distance towards a close friend of mine. I thought it had gone away, but it turns out it hasn't. Whenever I'm with her, I feel a sharp feeling of bitterness and I somehow feel like I want to just snap at her. I'm not quite sure why but I think she's somehow upset me. Or, I'm being a bitch but I've never actually really felt like this to anyone, especially someone as close to me as she is.