I recently hung out with a couple of friends. This friend doesn't know those other friends and plus I just hung out with her and she was working during the time I went out with the other friends as well so there was obviously no reason to bring her along. She kept saying that she felt "betrayed", and at first I thought she was joking because she's funny. But she's still constantly saying similar things like "what about me" and "you traded me." I can tell when she's not being sarcastic so that's why I'm wondering if this is something I should feel sorry about. When I her hanging out with other friends, I do feel left out sometimes but I never say things to sound jealous and always manage to be happy for her for having fun in the end. I don't want to jump to conclusions and assume so can someone give their take on this? Is it bad that she wanted herself and no one else to hang out with me? 

Since I'm a reasonable person, knowing the feeling of being left out, especially by her ironically, I assured her that we could hang out, but she's since left my message on open. I wanted to say she was petty, but held back to not judge too soon. But I can now boldly say that that's what she is indeed. She's sees that I hung out with other people and she can't handle it for some reason. Thank you for the advice.