In january 2021 i had kub ultrasound and everything was ok. now 3 days ago i had another kub ultrasound (mild pain in right flank) and a 13x15mm cyst is on my right kidney. i had a ct scan today and cyst was sphere shaped. why it happened suddenly?

Either the cyst was missed in January or it enlarges in the last 8 months. US images are very dependent by the technologist's skill and experience. If US and CT conclude the cyst as a simple cyst, you have nothing to worry. One simple cyst can be present at any age, the older the more common.

Several possibilities. Are you sure the "KUB" was an ultrasound and not a plain x-ray... (KUB is often used to describe these plain x-rays, and a cyst is not usually visible). 15mm or 1.5cm is on the small side. It is possible it could have been missed on original ultrasound, as it is very dependent on the person doing exam. Finally, I have my doubts that cyst this small,is cause of the pain.