I had to have some kidney ultrasounds, X-rays and cat scans when having problems with kidney stones, and the ultrasound scan showed a fair-sized cyst on one kidney, about which they didn't seem bothered at all. When I asked why, I was told that simple cysts in kidneys are fairly common and usually give no trouble and require no action. If you are having kidney pain and there is no obvious cause, they will doubtless want to keep an eye on any cysts you may have to make sure they are benign, and that may be why they have recalled you for a second scan. I wouldn't worry too much at this stage. Hope they find the cause, and that it's easily dealt with.