Sore sex

So my boyfriend gets sore most times we have sex. He said it’s an issue he’s never had before. We’ve both been checked for STIs and things but it’s all come back negative and he is going through further tests to find out what the soreness is but no one can tell us? Has anyone had this before? What could it be? Is it something to do with me or could it be him? He said it’s mainly around my period time so we have to avoid it a week before and after at the moment!