I got my first period at 16 but then it stopped for a couple years at around 30 they found out I have high testosterone which really meant nothing because I didn't have that much trouble conceiving when I was ready.
If there is no definite diagnosis from the doctor, ask for an abdominal MRI. Ultrasounds can't see soft tissue.
Try not to worry. While some girls do get their period around 12, some are older or younger. I got mine at almost 15! So, you'll need blood work to see your hormone levels, to check your thyroid and your pituitary. The ultrasound is to check on ovaries and eggs. I would try not to worry and just do these tests knowing most likely everything will be okay. Let us know what the doctor says after your ultrasound!