I been on my period for about 3 weeks im in pain and think I might have a cyst in my ovaier what do you think?

Heavy menstrula blee. Prolonged menstrual bleeding makes me think of fibroids, uterine polyps, or dysfuntional bleeding because you are not ovulating. It also matters if you are taking birth control or hormones. It is also important to check out your ovaries, as you suggested.