I just started my period, so why am I so tired?

Perhaps you are anemic. Low iron in the blood causes this, but if you are having your period, it can aggravate the condition. If you are tired to the point of not being able to function, then you should definitely get yourself to the Dr. They'll give you a blood test, and/or prescribe supplements or medication if the problem stems from some underlying problem.

It's normal. Some women get very tired with the onset of their periods. Take vitamins (don't overdose!) and get rest if you can. It should lift in a few days.

because your losing blood and iron.