Ammorhea

That should only be an issue if you're underweight.

No turns out thats not actually true. I had the same problem and that is what all the doctors told me. After weeks of reading journal articles from pub med I finally found a few that said things like "besides underweight athletes there are a group of heathy weight women who also have these issues, these remain to be studied". Basically its a fairly small group of us and they don't care. My body fat percentage was perfectly fine yet I still had problems. I found it got better as time went on. I missed about 4 periods in a row, then it came back but was a bit scattered, and then it regulated out to be fine. This isn't to say its not worth getting it checked out because its possible there could be something else causing it, but the rule of it being a purely weight issue isn't really true. Its just less common among heathy weight runners.