Actually...
You should be concerned about it! This happened to me and I was denied a first class medical. Had to go through a process with the regional medical administrator, in the long run it did turn out that their was something wrong with my kidneys, fortunately because it was caught very early, it was completely treatable. My nephrologist said he had never seen a case where this condition was diagnosed so early, and it was only because of the FAA medical requirement. Most people, he said, only discover it long after their kidneys are near failure. Also, once treatment was determined I had no problem getting my first class medical back.
As the previous poster said, it may simply only because you strained yourself (exercised) immediately before the medical. But my advice to you is get it checked out to save yourself heartache in the long run.
Best Regards,
APU