I do think that there is something going on with you and I think you really need to get a referral to a Gynocologist (as GP's just don't have the specialised skills to deal with Endo), plus the only way to confirm it is to have a Laparoscopy (which is a surgery that is done via keyholes in your abdomen and a camera is inserted to get a clear picture of what is going on in there). The sooner you get it sorted out and find out if there is anything going on (or not) the better as the longer you leave it, it can impact on your fertility when you decide to start a family.

Feel free to ask more questions if you have them.