You may just be feeling where the kidney is. One is usually lower than the other one, and it's right where the waistline is. If you feel it, that's normal. But since you say it's painful, might be a kidney stone trying to get thru, which this happens to some people. Drinking extra water will help this. Another thing that comes to mind, is constipation will cause swelling and stoppage of intestinal waste, so could be you're feeling that, lots of gas and constipation will make bowels puff out. Only other thing is since you had surgery recently, could be an infection has set up shop. Whatever it might be, you should see your doc, just in case it's something like a cyst growing on something in that vicinity, for example.