missing period for 3 months...?

Hi everyone, this is my first post here. I have always had irregular periods ever since I got it at 12, and they would sometimes come every month or so, other times come every month and a half. I'm now 18, and my period only came twice this year (february and april). It's been coming less for the past year or two, but it always came back, and I wasn't worried because I was still a minor. I haven't had a single drop of blood in 3 months, and I'm starting to get rightfully concerned. I have searched up possible causes and none seem to match up, apart from (perhaps?) exercise or low-ish body fat percentage. I am a ballet dancer and I dance a lot, about 3 hours of class a day plus rehearsals on some days, but my schedule has always been pretty full. I also lost about 4.5kg during the past 3 months (intentionally for ballet) and my BMI is at 18.2, which is on the lower side but not underweight. I don't think I have thyroid issues or PCOS, nor do I think I have excess stress. I know that a low body fat percentage will affect periods as well, but I still have a healthy amount (about 20% according to my scale and pictures online, although it's most likely not accurate). I also eat a lot of nutritional foods, my calorie intake is reasonable (I lost a little more than a kilo a month starting april, but now I maintain an incredibly slow and gradual weight loss so it's not like my calorie deficit is too large). I know I should go to the doctors and I will, but I was curious if anyone has gone through the same thing, and/or has any ideas on how to get it back? what do you think this issue is caused by?