It's probably because your body is about to spend the next 5-7 days working. It's kind of like if you saved $10 dollars a week, at the end of a month, you'll have $40 dollars, but during that time, you have no extra money. Maybe that's how our bodies work for periods. They save up energy to be able to do it's stuff, so beforehand you're tired.
Because you are losing blood.Why do i feel so tired before period?
Maybe your iron levels are low? You lose alot of iron during your period, you might want to think about taking iron supplements.Why do i feel tired before my period?