Is paranoia a normal reaction towards strangers?

Well, some guy came up to me at the library, and started to tell me how cute I was. Which made me feel REALLY uncomfortable because he looked WAY too old for me, like 16. I told him I was 13, but he still wouldnt leave, and to make matters he was 19 (that's not legal, right?). So then he asked for my phone number. I dont know what I was thinking but I gave it to him. Now I regret it. Now Im scared he'll come at my doorstep, or track me down. I doubt that'll happened, but I've been watching too many too many Law & Order:SVU episodes and one was about an older guy taking advantage of younger girls with his charms. I try to get a hold of myself but it isn't working. Maybe because I've never been hit on before.