Other reasons for no period besides PCOS?

Chelsea

My periods have been irregular ever since I got it at age 14, getting them usually only twice a year if I’m un/lucky. However, when I go on birth control, I do get it regularly, though it is heavy.

I recently went to obtain an ultrasound to see whether or not I have PCOS— though I’m not completely convinced I do. (Results are not in yet).

What could be some other reasons for having such an irregular period (besides hormone imbalance?)