My ultrasound showed a cyst on my right ovary but also said my uterus was heterogeneous. what does that mean and what causes it, if anything?

Not unusual: "heterogeneous" just means that the uterine wall has some areas that look different from other areas on imaging (as opposed to "homogeneous, " which means that the whole uterus looks the same across the board). Sometimes it means that a woman has fibroids or adenomyosis, but it is usually not concerning. Your doc can tell you how that finding applies to you.