Why do people faint??

Okay, so a few weeks ago I was sitting in the Online P.R lecture when I looked at my arm and I had a rash all over it, I showed it to Steve and Paddy who informed me that it was probably some kind of disease that would end up killing me in some way... thanks guys! Then I got a really bad headache so I went up to the nurse with Megan and got checked out, she told me I should go home to bed and try sleep it off. So I got the one o'clock bus, we were going through Dunleer when everything started passing by me really slowly and cars that were doing 80kph seemed to be crawling along and only doing 10kph. I knew I was going to faint, I put my head in my hands and the next thing I remember is waking up and some boy asking if I needed something sugary and was i diabetic, and no I'm not diabetic, I just always faint! He called the girl who had been sitting in front of him and she came over, she was really nice and gave me water in case I was dehydrated and let me use her phone to ring my Dad. I faint at least three times a year and I have apparently been fainting since I was four. So what I want to know is why people faint?? Is it some genetic thing, because my Mam used to faint a lot when she was my age. When I told my Grandad about this he said to me 'Ahh sure it was probably something to do with the weather,' what is it with Irish people and assuming that when you feel sick or have a cold or in my case faint it is because of 'the weather'?? Is it just an Irish thing or is this worldwide?? And is it true??