Why do i feel so tired?

i was at school and it was about 2.40 i was just playing on the keyboard when suddenly this huge amount of fatigue smacked me in the face. i am 14, a girl, 158 cm (5 ft 2) high and 52 kgs (114 lbs) and a vegan. that day i had eaten special k and oat milk for breakfast, a small about of tomato soup and beetroot potato spinach sweetcorn salad for lunch. i have not been diagnosed with anemia. when this happened to me i thought it could have been low blood sugar so i had a can of soda but felt no better. when i got home i tried sugar again and had a muffin but no change. i also slept for about an hour but i still feel really tired. once i had this at time of the month but i think im due for a period later in the moth. i had sufficent sleep last night and the night before and had enough excersize yesterday but not really today becuase i got so tired. but why did it happen? what should i do?