Why does it burn when i pee?

It burns like EVERY TIME i pee!! it's not and STD or an infection of any kind, and it isn't the soap i use, either, my doctor checked! so whY?? and also sometimes if the pee won't come out, i get these cramps one side or the other of my lower back sort of where my kidneys are and they hurt really bad! this has been going on for awhile???