Yes, you are definately being paranoid. Every girl goes through this the first time she has sex. It's because our parents have threatened us with pregnancy if we have sex. I went through this very thing when I lost my virginity. It also caused my period to be late and that really scared me. But I was fine and I didn't get pregnant until I wanted to.
I will not preach to you about having sex, but I will say this: don't do it if you're not ready. If you'll feel this scared every time, then it's not worth it. Also, you want to think about what you'd do if you did get pregnant. If you don't feel like that's something you could do, then not having sex is definately the safest birth control. So, if you do plan on remaining sexually active, go ahead and get some birth control. I promise, you're fine. If he was wearing a condom and only put it in you for a little while, the precum couldn't have even hurt you.
Feel free to discuss any more concerns
don't worry, ok?