Left Out

I am currently in school, and things are happening that make me feel left out an like an outcast. I'm not sure if I am over reacting or if I just feel betrayed because my best friend is hanging out with my ex best friend... but I'm not quite sure what i am feeling right now. I am over reacting I guess because I have no problem with them hanging out normally but with them hanging out together in front of me reminds me of when we were fighting and it was the two of them against me and I guess it just brings back memories and those memories scare me.