I always get lightheaded and faint when learning about sex ed, can anyone please tell me why this happens?

I am 13 yrs old and im just finishing yr 9 at secondary school. But i have a problem that is big to me and that problem is i always feel very weak and go lightheaded when learning about sex education or about the human body in depth(as you can see they are linked together). I have fainted twice at school; One being in a sex ed lesson and the other in a science lesson about the insides of a human body.When i fainted in the sex ed lesson i got a 2 1/2 graze, glowing red just above the top of my left eye and a little cut just below my left eye so you can see i was very lucky to have missed any harm to my eye. I got the cut because i was sitting down on a chair and i fainted to my left which caused me to hit the edge of the table and then hit the floor(amazingly i didint feel any pain because i blacked out for that moment). My other faint was not to bad as i escaped clean injury-free but it was just over a few facts about the insides of a human body! it was very hot though so that could be an excuse ;) But on a serious note, can anybody please explain why this happens to me and give me advice on what to do to resolve this problem i have? All comments will be appreciated and i will feel very humbled and grateful :)