My best friend.?

I asked her if she wanted to se fergie at the rodeo and she said maybe. three weeks later she told me no because she had to work. I said ok and didnt ask anyone else because shes the only one i wanted to go with. well the concert was last week. the day after the concert my friend posted pics on myspace of her and some chick at fergie. so should i be pissed at her? She said it was mandatory for her to work on that day. Shes also gay. so this chick might have been a love interest