Period misses...

So, this is my first time posting, and I'm doing it cause I'm worried...I haven't had sex yet, but I've also missed my last 3 periods...my dr says what's happening is called Pearling...I'll explain what that means if you want...but what I wanna know is, how do I stop this from happening? How can I make it better? My dr says exercise and healthy eating, but I already do that...