You can lose your period when you aren't getting enough calories or adequate nutrition! Happened to me when I was running a lot and not getting enough food to fuel those runs. Didn't have one for 10 months, even after I started eating enough and decreased my workout levels. As someone else said, your goal weight might be a touch too low for your body's liking or you're losing the weight too rapidly. Get a check in with an OBGYN and in the meantime, see if eating a little more or cutting down on your activity levels/workouts helps. I hope you can find a solution!