What Causes Fluid In Uterus?

Hi, I'm a post-menopausal woman (72) in good health. I've been having pain in my left side and just ha a pelvic and abdominal ultrasound. It showed small renal cysts and fluid in my uterus. I'm wondering what causes the fluid. My doc said the cysts are very small and do not require medical intervention so I'm assuming it is the fluid that is causing my pain. Thank you.