What to do if I have 2cm renal cortical cyst, could this couse of back pain?

No, usually not... Cortical cyst(s) inside kidneys are common and they tends to become bigger with variable pace over time but causing no pain at all until the extremely rare occurrence of its internal infection / bleeding. So, look into other sources for chronic backache, which more commonly results from musculoskeletal or neuromuscular origin. Detail? Ask Doc timely.