Several causes. Fluid build-up (called ascites) in the peritoneal cavity (abdomen) can lead to bloating. Tumor cells lining the intestinal tract and other organs can also lead to bloating.
Probably ascites. Ascites is a common finding/complication in ovarian cancer patients. Bloating could also come from the increased size in the ovary due to the tumor, which results in an increased abdominal girth. It could also be from a condition called endometriosis, which can make fluid and cycle like a period which can also cause bloating. Estrogen replacement therapy increases ovarian cancer risk.