I have looked around the internet. Answer for everything right? Now i cannot say for sure. Me and my girl had this problem at one point. But it may not be the same deal. One problem could be that you were “getting bored (not being as turned on).” Thus making it rougher on you, and vice versa him, if he even noticed. Another problem could be that it is simple you guys were doing it for so long. Many solutions online say simply use lubrication at this point. I have not dealt with the situation about entry, and it hurting. But if you do wait to long i have read that you could actually shrink a little if you have prolonged periods of no sex. But the obvious one could be you are not turned on enough. Otherwise try lube, another method is try different positions, that way its not continuous one position.  Or a combination would be best. 

-Michael