Im 20 yrs old and had a ct scan for a kidney stone and they found a incidental 11 mm interpolar left renal cortical cyst should i be worrried?

Renal Cyst: Renal cysts are very common , often benign and asymptotic fluid filled structures of the kidney's. Sometimes they can contain blood and can be difficult to better characterize on imaging. If they are only thin walled structures usually need no further work up, if they have solid portions may need follow up.

No,: this is a very common incidental finding. A small renal cyst will generally cause no symptoms and require no treatment.