What Causes Yellow Vaginal Discharge While Using Copper IUD?

Hi, im 27 years old and im sexually active. I get my period on time as well. I'm using birth control, a copper IUD. For the passed few months i have been having very bad discharge which is yellow. I was not aware that it was bad until my bf told he was having some sharp pain when he wants to urine and some yellowish discharge as well. We both consulted a gynecologist. It was an infection we both got treated and i even did a pap smear. The results came back as the normal infection and NOT STD. I was given some medication as well. This was about 5 to 6 months back. even since then, after my period i still get the yellowish discharge. It has slight odour, Its like gel, bright yellowish and now i get very bad cramps like period cramps with slight back pain. I feel like there is always liquid coming out. Could you please advised me on what may be the problem? is the cause the IUD? Appreciate your feedback on this. Thank you..