kidney infections
"I had just taken Diflucan for what I thought was a yeast infection. Then I got a sinus infection and I started taking cephalexin. I noticed 3 days in, some back pain (flank pain just to the left if my spine) I had been having for a few weeks suddenly went away. I went back to the doctor, who agreed that I probably originally had a bladder infection that spread to one of my kidneys. Glad I got the sinus infection or who knows what would have happened. The medication made me feel a little nauseated after the first few days, but it's possible that feeling was related to the sickness."