Why do I feel this way

I have been seeing this guys for a while a couple weeks now, we have been good friends for almost 3 years and once he broke it off with his girlfriend we started fucking. I am kinda talking to other guys too rn and I never assumed we were exclusive. Since I am not 13 so I'm smart enough not ask questions I won't like the answer too, like are you still talking to your ex. I have heard rumors they are but it never really bothered me until today. My friend took a picture of his exes year book (we got them today) and he wrote like 3 paragraphs talking about how special she is and how much she means to him and how she made his junior year so incredible. After reading it I made up some bs excuse on why I had to go home and now I am sitting over a toilet about to throw up. My confidence is shattered I can't eat anything and i feel so bad about my self. I don't know why I am having such a physical response over something so dumb. I am talking to other guys too, I don't think about a future together or anything. Wtf if going on with me. I am supposed to leave to go party and then inevitably hook up with him in like an hour. Idk if I can do it. How do I make my self normal again.