I lost my period very early in my disorder, when my BMI was still technically in the healthy range. The situation was very similar to yours: I was a vegetarian, was eating less, and doing daily exercise. I think maybe I was burning off a lot of fat, so even though my weight was okay, my body fat percentage was low.

You can always go to a doctor if you're worried. Everyone had different bodies, and for some people, having irregular periods just happens sometimes for no reason and is nothing to worry about, but if you feel like something is wrong, it's always best to get it taken care of. You probably won't be diagnosed with anorexia (I think they normally say three missed periods for that), but they may want you to increase your calories.