The process of micturition, the actual release of urine from the body, is more than simply relaxing the sphincters of the bladder. In order to generate more pressure, and thus a stronger stream, your body will also tense the muscles around the abdominal cavity. This is likely adding pressure to gas already in your system and causing your symptoms.
If you are female, it is possible that you may have an ovarian cyst that could be causing this discomfort. Another issue that you may need to look into may be for gallstones.
I could be due to several reasons. I would check with your doctor to have it evaluated. It could be caused by kidney stones, urinary tract infection, kidney infection among many other things. Check with your doctor to be evaluated.