What Causes Pelvis Pain With Burning Sensations?

Hi, I have been experiencing a pain on my left side pelvis for nearly 6-8 months now. On doing a CUE, it was found that I had about 15-20 pus cells/HPF for which I took medication for 10 days. I repeated the CUE and found that pus cells had reduced to 1-2/HPF. But the low throbbing pain has not gone away and I have a burning sensation always. I did a USG to check if the urinary bladder and urethra were fine and there were no issues. No kidney stones could be detected. What is the cause of this problem?