Help!!! Pain During Sex

So my boyfriend and I just recently started having sex a couple months ago. I never felt any pain with my past two partners, but with my current bf, I do. It mainly happens when I'm on top, but not always and it hurts like hell. It's gotten where I'm scared to have sex again because I'm scared of the pain, and when we do have sex I cut it short because it just hurts too much sometimes. Any ideas on what might be causing the pain? Any ideas on what may help prevent it?