What Causes Secondary Amenorrhea?

Hi. I am 19 and only ever had 3 periods. I have been to seen 2 specialists and they both said it was because of my weight. However, I have friends that are the same age and height as me and they are a lot bigger than me in weight, but they have periods every month. I went to see my doctor recently and she said it was secondary amenorrhoea. My hormone levels are normal and I ve had a few stomach scans and everything is normal there. I have been tested for polysytic ovories but test results showed I don t have it. What could it be?