Reasons you won't talk to a guy?

I didn't mention anything about sex did I ?

This girl she just seems to be less interacting with me. She will tend to ignore me in a group especially when another girl is around, she won't say anything, if she does it's rare. With another guy she's all friendly and even a little flirty and quite comfortable with him, but me she just won't start conversations with me, yet when she's talking in a group she will look at me when she's talking. She rarely looks me in the eye when walking past, rarely smiles, but I don't really smile at her.. Yet if it's one to one and she feels comfortable she will give good eye contact, be polite, she even opens doors for me when my hands are full. ( I always open for her) I've noticed she quickly glances at me sometimes. If there's a problem she very often comes shouting for me. I'm confused at to what I may have done, she used to be totally different yet now it's completely different.
Do you think I've offended her? The only difference is that there's another girl who I get on well with, and this is said girl that when were together she will barely talk.