For someone who is dieting, the minimum amount you should consume is 1200 calories. Anything below that, and it's unhealthy. The "normal" person's minimum is more than 1200, it depends on the individual's sex, height, and weight.

My question wouldn't be so much the calories, but the attitude you have with eating. Are you purposely eating this many calories, or is this normal for you? Are you trying to lose weight? If you are restricting and trying to lose weight, I would say that this is a definite issue. If not, then I'd suggest trying to increase the amount of calories you consume, so you can gradually put some weight back on.

The symptoms of not menstruating and hair falling out are troubling as well. Would you be willing to see a professional for your concerns?