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?