The problem you describe, of feeling you need to empty your bowels during sex, is not uncommon but there should be no cause for concern.
There is a very thin wall between the vagina and the rectum and when you have sex, your boyfriend's penis is pressing on the nerves of your rectum which gives the indication that you need to go to the bathroom.
It is nothing to worry about but, because you know it happens, you are thinking about it instead of enjoying the moment, and wondering if it will happen will only make it worse.
Try to relax and enjoy the intimate connection, there is no reason why you would lose control of your bowels, it is just a strange sensation.
You could try different positions, maybe there is one that relieves the pressure and makes you feel more relaxed.
Just to be safe, visit the bathroom before sex, if nothing comes out then, it is safe to assume that there is no chance it will make an appearance during sex either :-)
