Reasons you won't talk to a guy?

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 she quiet. 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.
Another note if I'm talking one to one p, she will keep good eye contact and almost always be touching her face somewhere. Even other people have noticed her change in behaviour towards me.

You might make her nervous and that could be for a few reasons, one being she likes you and might not want you to notice she does so she avoids you or eye contact with you. I don't know though for sure I can only assume here. The only reasons I wouldn't talk to a guy is if he did something wrong to a GF and I heard the rumors and they were found to be truthful, I would ignore him for her or if he was rude to me previously. But some girls who are shy will avoid guys they like.

Girls are crazy jealous....if they are not getting the attention they are used to and see it go to someone else, they get all weird. It doesn't mean they have a crush on you or whatever......some girls are competitive by nature, and when someone new comes along and guys focus on the new girl, all the other girls get catty and start turning into snobs.

If you like her, ask her out and then if she agrees to go you know she likes you too, because if she didn't she'd never accept the invite.

I wouldn't talk to a guy if I felt he was a douche bag and a history of being one or a player and rude. No time for those guys.