Why does sex hurt?

It hurts after I have sex with my BF in my vagina area like inside of it. I figured maybe he hit my cervix too hard/ too much but he's only 6 inches long. I don't know if that's normal? It's been hurting since we've started having sex again last month and it feels crampy and its off and on pain. I know you guys aren't doctors but does anyone have experience with this or know what it could be? Thank u!