My best friend is upset with me over Sweeney Todd.?

I called my friend, Mimi to tell her something, but when she answered the phone she sounded really angry. I thought it might have been a family thing cause the tone in her voice was that of someone who couldn't/didn't want to talk at the moment. I tried to get off the line after I told her what I had called to tell her, but before I could hang up, she told me flat out that she was angry at me. The last time we had talked, I had told her that I would be seeing Sweeney Todd for the first time with my other friend Andy on the new premiere date(21st). Mimi and I had agreed about a month ago that we would see it for the first time together on the 26th when she would be here to visit for the holidays (she lives in another state), but a while ago she told me that she might not visit. After she told me Andy asked if I wanted to go with her, and I agreed. When I told her that I would be seeing it for the first time with Andy she didn't seem to mind, but now she's mad at me. What should I do?