Could be he is running into your cervix. It is not uncommon an can be painful. When you are not aroused it is only about a finger's length from your vaginal opening, and feels soft and bony like a nose tip. When you get aroused the vagina lengthens, your cervix moves up and allows for deeper penetration. If you are constipated it could also be hard stool, since there is only a thin membrane between the vagina and anus.