Why is sex painful lately?

I've been having sex with my boyfriend like normal lately, I'm used to smaller partners but my boyfriend is well endowed and after 5 months of dating sex has become progressively more painful. My boyfriend looked at my vagina after I told him It hurt too much to have sex (it feels like a tear, the pain extends to my butthole and sometimes creates a burning sensation) he told me it looked swollen and it does look like there is a tear (could it be a slight perineum tear?) and there is like an extra piece of skin which is small. I have been tested for std's and they've all come back negative, what could this be?