Hi, After Standing For Over 15 Hours, I Started Having

Hi, after standing for over 15 hours, i started having moderate to severe pain in the lower right side of my back.
this is the second time in the past 6 months. It doesn't hurt while standing, mainly when laying on my side. I had an ultrasound of my kidneys and badder in June an was clear. Can it just be musculature? I am overweight. Your thoughts please.