I Am Having Pretty Strong Pain In The Left Side.

I am having pretty strong pain in the left side. It hurts pretty constant for second day straight and seems to be getting worse. I initially seemed to get some relief with a heating pad but now no relief. I took a Naproxen about 1/2 hour ago (220) mg. and little to no relief yet. I thought it could be a kidney infection. I feel it in my back, side and stomach. It is rather a large area though. If it is not a kidney disease, what else could it be?