What is wrong with me? getting so jealous of friends' relationship with other?

So basically, when my two friends who used to be best friend get together, I get really jealous. And when O said get together, I mean things like hanging out together without me, or even just simply talking on Facebook. I get so insecure, I'm scared that they will ditch me one day, or that they are bitching about me, or maybe they are planning to go somewhere without me. I'm so insecure that even when I talk to them, and they mention each others' name, Ii feel like something is wrong. And also, I realised that I try to hide the fact that I'm good friends with them. Like I don't want A to know that I'm good friends with B, and I don't want B to know that I'm good friends with A. But I get so upset to see that A and B are so close. What is wrong with me?