Do you have a nephrologist? Do you have an autoimmune disease? Are you dehyrated or have you done unusual exercise or lack thereof? I get a Cystatin C test- a different kind of eGFR- that is more stable and reassuring. The same drop happened to me two years ago and I still don't know why but my eGFR is now stable. I avoid NSAIDS, contrast and other things that affect kidneys.