Yes, this is very common. I lost my period for 9 months the only time in my life I was ever skinny enough to wear a bikini. It can happen either with too much weight loss or too much exercise. It's called Exercise Induced Amenorrhea and it's not healthy. Normally women have a period ever 28-32 days. In the old times women had fewer periods because they breast fed for many years after pregnancy and were pregnant a lot more times in their lives than they are now. You can still get pregnant without a period but it's less likely. Exercise Induced Amenorrhea is not a birth control method at all. You don't get a free pass because your girlfriend has amenorrhea.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 180 x 2. Current Deadlift: 230 x 2