As weird as this might sound, it could very well be an allergic reaction to your husbands ejaculate. After time and enough exposure ones body may very well develop a reaction to certain things. It's not actually all that uncommon. I would bring it up with your gyno on your next visit! Just one possibility!
Hi Chick, So when iy comes to sex you need to find a postion that is best for you with your pelvis aching, as for the burning, have you thought of getting to wear condoms, I know us men dont like them, but if it stops the burning, as he should be thinking of you and not him, and see if that stops the burning, if turns out to be right then get him tested.
Good Luck
There could be several different reasons why this is happening. I wish I could give you and answer but, I'm just not sure. Have you seen your Gynecologist about what's going on? If not, I would recommend doing so.