Despite being on birth control, I've been bleeding and cramping for ten weeks. Thyroid levels are normal, and not preg. What could be causing this?

Polyp or fibroid. When women take birth control pills, usually the bleeding is regular. There are hormonal reasons why bleeding occurs like that and sometimes the pills thin out the lining of the uterus and cause that, but first i would recommend making sure there is nothing anatomical causing the bleeding. Uterine fibroids and polyps can cause that. Depending on your age, sometimes a quick office biopsy of the uterine lining is warranted as well.
Abnormal hormones. There are various levels and types of hormones in birth control pills and sometimes different brands need to be tried to get the right combination. Othertimes problems can be present like infections, fibroids, adenomyosis, polyps, etc and further diagnostic tests and ultrasound may need to be performed.