The question about hormones is the first thing I would wonder, also. Call your doctor and ask if your progesterone levels were tested. People bleed when they have had high progesterone and then it drops. Maybe yours is off kilter.
Also, when they did the ultrasound, did they see any fibroids? I would ask for a uterine MRI, next.
Did the birth control help with the period? Are you more regular now? Did you have any hormones tested with the blood work?