Firstly, don't stress. That discharge sounds normal as long as it isn't smelly or bloody.
Secondly, it sounds as though you are having sex at an odd angle that might be hitting your cervix or something else.
Thirdly, PLEASE USE CONDOMS. At 16 years old, you could potentially ruin your life if you get an STD that is life threatening. You also don't want the added stress of a pregnancy.
Look after yourself and make sure you communicate the pain to your boyfriend when you're having sex.
Remember also to go and see a Dr if you are in doubt with anything you are feeling