Irritated.

So at work I got a (sort of) promotion. I didn't ask for it. I didn't even want it at first. My boss pretty much forced it on me. Now I'm in the role, it's not what I expected and I like it. But it's caused a problem between me and one of my co-workers. She was expecting to get it. She kinda campaigned for it. I found this out today when my boss told me she might be a little standoffish to me because of it. Should I be offended that she is so upset at me? I don't think I did anything wrong. For whatever reason, management felt I was better in this role than she was and told me they wanted me in the position. I'm not sure how to act around her now. I don't want her to be mad at me over something I feel wasn't my fault. It's irritating when people act so immature.