Pain after sex

Hi was wondering if anyone could shed any light on my current situation. About a year and a half ago after sex (unprotected) I started feeling a burning sensation in my lower abdomen. The feeling persisted so I went and got tested for all stds, which came back negative. I then left a urine sample into the doctors and they treated me for a uti with a course of trimethoprim which cleared up the pain. However, every time I had unprotected sex the pain came back and I had to go back to get antibiotics from the doctor. The pain only came back when I had unprotected sex (not protected)

However, in september I left a urine sample into the doctor to get the course of antibiotics and I was told that I didn’t have a uti and that all the previous urine samples I had left in came back negative for a uti.

So I was obviously left confused, I was referred for an ultrasound scan which didn’t show much except a slight enlargement of one of my kidneys.

I also saw a gynaecologist in December who is of the opinion that it is not gynaecological, however I am awaiting a laprascopy.

I went back to the doctors last month as I started to feel as though I had a uti again (burning when urinating etc) and he prescribed me a low dose of trimethoprim for 3 months. It’s help a bit but hasn’t cleared up the pain.

I was wondering if anyone has any ideas what the pain could be or why the trimethoprim got rid of the pain? The doctors don’t seem to know much at all. I’ve been tested for stis 4 times in the past year and all are negative so I know it’s not a sexual infection but confused as the pain initially started after sex.