I would definately go back to the doctors and press for more examinations...have they checked your cervix to see if there is any inflamation?
i dont know how old you are but maybe you could ask for a pap smear, just to be certain. in the uk it is now encouraged to see a doctor if you experience any bleeding in between periods or pain during sex. (seen as they wont reduce the testing age to 20)
Its worth to, not worry about it too much, but keep everything in mind. you know your body and you know there is something wrong, so dont give up till you get answers.