unexplained pain

My bf and I had a third base interaction a couple days ago. we tried again today and encountered a problem. it literally feels like he's got a bit of sand paper down there, just in the entrance bit tho. once he gets past there everything is all good. it's just the incessant in and out action hurts because of the sandy paper feeling. and I'm perfectly wet and happy. I'm not sure what's going on, we both have got clean std tests and everything. maybe female form of blue balls???