anal pain when sitting down

I have been having anal pain when I sit down for about two weeks there is no anal bleeding but during my bowel movements at the begining it is painful in my lower belly my boyfriend and I do have anal sex it has been about a month or so since we usually use lots of lubricant and he is very gentle the last time we did it, it was spur of the moment and we didnt lubricate could this be why if I feel this pain it sort of feels like a cramp there is no burning and there is nothing visible on the outside can someone tell me what this could be?