What Does Decline Of GFR From 58 To 51 Indicate?

I recently found out that my GFR is 51(down from 58 in April) is that pace of decline surprising? I am 51 years old, in generally good health with no diabetes or drinking problem. My internist did not even notify me that my GFR was out of range but I discovered it when I needed a copy of my labs for other purposes and then checked my labs from April when I saw the red flag .