Drop in GFR Despite Healthy Lifestyle

I’m puzzled by a sudden drop in my GFR from 90 to 60 over the past six months. My creatinine levels also went from 89 to 66. I’ve been eating healthy, staying hydrated, and doing CrossFit, but nothing seems to help. I’m older but feel fine overall, except I now need to get up once or twice at night to urinate. Any ideas on what might be going on? Thanks!