Sex hurting!? Help!

Hey girls! Quick question: sometimes when I'm at my boyfriends place and we have had sex once or twice already it can get really painful, sometimes so painful that even though I say he can keep going he stops because I 'look like I'm about to cry' .. Is this normal?! I mean we can be pretty rough and definitley don't stick to normal positions or anything so would this just be because we aren't being gentle? (I'm 16) and in sex ed yesterday practically every STI caused painful sex and even though my boyfriend has been tested and clean and I'm definitley okay I'm like.. Kinda worried something is wrong! Help!!! Does this happen to normal people? (All my friends are virgins and my mum will disown me if she knows I've had sex even though it's with an incredibly caring and kind guy..)