Mid 30s and told from ct scan w/ contrast that i have a "low density" 6mm lesion on my kidney that they believe is a cyst but too small to characterize. ultrasound after saw nothing. really worried. is this cancer?

Renal cysts are relatively common. An estimated 10% of the entire population have simple renal cysts. Simple renal cysts, when asymptomatic, are typically not treated unless there is an infection, concern for malignancy, or other complication. Unfortunately, there's no way to know for sure whether or not it is malignant without surgical intervention.