Feeling faint after running?

well i remember in school we did things outside. we were doing relay-races and i was in running.. and i ran really fast.. and afterwards i felt faint and things started to get blurry slowly.. we did these races after lunch.. luckily i drank water and eat some ice and that made me back to normal.. wat i dont get is y did i feel faint.. i mean i never felt like that after running and it feels like everytime im active that happens..i felt the same way after jumproping ..all i usually do is stay inside my room on the computer.. could that affect it?