Why do i feel upset about my friends?

So I hang out with one of my friend a while ago. And my other friend found out and she got angry because we didn't invite her. I told her that it was on the last minute but she doesn't believe me. Things went a bit awkward after that day. Although we still talk like we used to, I could feel the awkwardness. And lately, she went out with the friend I went out with before, without inviting me. I don't know why, but I feel jealous. I keep telling myself that although they didn't invite me,we are all still friends. But I don't know why I feel like they have been talking behind my back and might hate me or not want to be friends with me. I never felt this way before. But now, I'm worrying so much. I feel like they went out without me and talk about how much they hate me all the times. I told myself that as long as we are friends everything it's fine. But I just can't stop worrying. Can someone please give me some advice?