What could be wrong with me if my kidneys hurt on the left side?

Infection or stone: Not to be rude (you should only have one kidney on each side), but how do you know it's your left kidney that's hurting? Flank & back pain could simply be due to muscle strain. Or it could be due to kidney stone or infection. Or a stone leading to obstruction leading to hydronephrosis. Heck, maybe shingles or hematoma. Bottom line, go see your doc for an evaluation, esp b/c warfarin. Good luck.