I'm 18/female and I feel sick/uncomfortable at the thought of dating. What is going on?

I've liked my fair share of guys (okay probably more than a fair share) but recently, I've realized that whenever I like someone and/or find out that they like me too, I feel sick to my stomach. Not like I'm going to throw up but there's just an uncomfortable pit and I feel like I can't breathe. This has happened before when I found out that this guy liked me and was going to ask me to prom. I immediately rejected it (through friends) even though he was completely nice and not even pressuring in any way. Just recently, there was this guy I met and started talking to. I thought he was cute but I didn't really do anything about it. Instead, he contacted me and tries to talk with me sometimes. Apparently, he had even walked over to where I was studying with my friends. (maybe he ran, I heard he was sweating). Maybe I'm freaking myself out. Maybe this isn't real and I'm just being paranoid. I want to be in a happy relationship but every time it becomes possible, I freak myself out like this. Someone explain please. (I'm 18 and have never dated before)