Why do i feel jealous? Is it normal to feel like this?

A while ago, my friend and I went shopping together without inviting another friend. That other friend found out and got really angry at me because she felt excluded. I told her that it wasn't planned but she doesn't believe me. Things went quite awkward after that. We still talk like we used to, but I could feel that we aren't as close as we were before.

She didn't get angry at my other friend who went shopping with me though. Actually they went to a party together without me. I don't know why, but I feel jealous. I never felt like this before. I keep telling myself that although they are really close, I can still be their friends. But I feel like they are drafting away from me. I feel like they talk behind me and they secretly hate me. I don't know if it's true but this feeling keeps coming to me. I can't stop thinking about it. I simply want to be friends with them but they are so close with each other. If I tell one of them something, the other will definitely know. Even if one of them is lying, the other one won't trust me. I'm getting very stress now. People tell me to stop worrying but I just can't stop. I tell myself as long as we are friends everything is okay but I just can't stop thinking that they hate me deep inside and although they treat me like a friend in front of me, they talk about me when I'm not around