Pain when getting fingered?!?!

Sorry for the tmi but lately when my SO and I are getting "ready" and he starts fingering me it hurts a lot. It feels like he's sticking a knife in there. It's also completely random. Sometimes it hurts and sometimes it doesn't. At first I thought it was me maybe not being wet enough that got ruled out and then I thought maybe he's got long fingernails but it's not that either. I'm really stumped and not only does it really suck but I can tell he gets upset when I ask him to stop cause he's hurting me and now he doesn't want to do anything down there cause he is afraid he's gonna hurt me. I'm really stumped about what's wrong. Any advice?