Why do i vomit/feel nausea and then pass out?

[madzbk99]

So over the past year there have been quite a few random times where i will feel sick and most of the time throw up and following that i would pass out either straight after or a few hours later when i would feel sick again. i'm only 18 years old and have no previous medical diagnosis of anything and was hoping someone may give me an answer as to why this happens been as its gotten to the point where the last time this happened i had to get off the bus i was on because i felt like i was going to get sick and passed out in the middle of the street which is quite dangerous luckily there were some people who kindly helped me.