Why does hurt?

I am 18 years old and I am having for about 2 years. The first time and a few time after were very painfull, but oki since it was at the beginning. So I had a few times with this guy and after we split up I started dating another guy and I had with him too. After he cheated on me, I split up with him, and I had only after 6 moths once, the after 10 months once and now with my boyfriend. The problem is that sometimes it feels good but usualy it hurts (I never had an by having ..) especialy when I'm on top.. it feels like he hasn't enough space.. (I don't know how to explain it...) And sometimes it hurts even after I had , for 2-3 days and it hurts even when I pee, it stings. So my question is why does it hurt? At first I thought it hurted because I was a virgin, but why does it hurt now? Is it because I have very rarely?