Why do I get drunk so easily?

Back in highschool, I used to drink a lot. The usual teenage drinking, only a little bit worse. I’d get very drunk, very often, and the amount of alcohol I had to consume in order to get drunk got bigger and bigger. Then I had this one really bad incident and stopped for good, because I realised that I was exposing myself to lots of dangerous situations. And a friend of mine almost died of drinking.
Anyway, it’s been almost a year and a half since I quit drinking for good, I took up boxing a few months ago, have been living a way healthier life ever since. But: I can’t drink more than a few sips of beer (for crying out loud) without getting tipsy and giggling and getting an upset stomach.
So the question is: why is my alcohol tolerance so incredibly low now? It wasn’t like this to begin with, like, ever.