Anaemia (basically a lack of iron) could cause this but the dr should be able to check this with a simple blood test!!

Some women & girls lose alot of blood when they have a period & so just don't have enough blood left until their body has time to "fill up". Again the dr should be able to check this.

Try (& this won't work like magic it will take time to build you up & some women just don't, iron passes through me like water) eating iron rich foods (any leafy green vegetables, tomato, parsley etc) or taking an iron supplement. Try to get plenty of rest just before & during your period. You may need to have a few days off school each month, at least until your body adjusts.

Good luck