I am a female and I'm 21 years old. I'm 5' and about 110 lbs.I started drinking when I was 15. Back then I could only have about 3 beers and that would get me completely drunk, after that I would have a few drinks and black out. I would drink almost every weekend until about the age of 19. After a very embarrassing incident I decided to stop drinking and I did until last night. Given the fact that I am 21 and live at my university, I decided I would join a few friends in some shots. I only had 4 and I was completely wasted and blacked out. 

Why is it every time I drink I black out?