Hi i've been having 2 periods a month,and they gave me an ultrasound,nothing was found but a 3cm cyst,my dr.put me on birth control,should more test?

No: If there was no evidence of a fibroid or polyp as cause for bleeding, then likely hormonal and your doctor was correct in giving you hormones to treat abnormal hormones. Sometimes can check thyroid. Are you extra thin or heavy? That can be a cause as well. Good luck!

Recheck ultrasound: Your GYN will likely recheck an ultrasound in 2-3 months to make sure the cyst is gone. No other tests are needed at this time. Your risk of anything bad, like an ovarian cancer, is extremely low.