Had ultra sound 3 times on my ovary. left always very enlarged, but no cysts. it's causing discomfort, as i can feel it. what could this be?

See MD ASAP: is there a family history of cancers ? see gyne onc MD ASAP with serial US pictures may do blood tests also

Not likely cancer: 3 ultrasounds and no change, sounds stable, Ovary cancer at 31 very unusual. If lots of pain and worse with periods more likely endometriosis. This may require a surgical investigation. Regrettably blood tumor markers are also elevated by endometriosis.