Played some tackle football for fun. Flank is sore. How do you know muscle from kidney trauma?

You may not know. Unless urine color changes reflecting poss bleed. A doctor can tell by ultrasound that may show a bruised kidney or by microscopic blood in urinalysis. Rarely, severe effort in hot weather especially in a non conitioned person may cause muscle breakdown with pain and muscle products that pass through kidneys may cause pain. This condition is called rhabdomyolysis. If you are better, do nothing.