@gingerw & @collegeprof have explained eGFR and the Cystatin C values beautifully. They also highlighted the importance of overall trend’s versus single tests in time. They are also correct about folks with CKD who would be thrilled with your numbers. I have stage 4 CKD and my eGFR is in the low 30’s.
Since your hospitalization in February with an acute kidney illness and then a “mystery” illness since March you continue to have symptoms of “constant nausea, extreme weakness and fatigue, back pain, urinating every 10 - 15 minutes. But no fever, normal UA, and only elevated neutrophils / immature granulocytes in CBC” ?
Have you asked your provider what the cause of these symptoms could be? Your situation sounds miserable. I hope you get some answers soon that will lead to helpful treatment.