117kg, 22 years old, normal egfr. sometimes, sudden low urine output, 0.5 ml in 12 hours with 2 l water intake, dark urine. fixed after 1-3 days. sometimes protein in the urine but it goes away. one time low urine lasted 8 days. what can it be?

What you have experienced is strange, not physiologic. If GFR is normal, urine output will largely depends on the total fluid intake unless you lose so much fluid from insensible sources such as sweating. If I was your doctor, I'd be interested in checking your blood and urine on day you have very low output although I am not aware of any specific condition which causes variable output as yours.

The data you provided are confusing. I assume you meant urine output of 0.5L in 12 hours, which is actually within normal limit. 24 hours urine output less than 0.5L is considered oliguria (low urine output).