sudden alcohol intolerance

in the past year or so i gained an abrupt intolerance for alcohol of any kind. i started drinking a lot when i was 13. i never had issues with hangovers and rarely experienced stomach problems. when i became a freshman i heavily drank multiple times a week no problem. at one point i drank so much my heart stopped beating and i had to get sent to the emergency room. the reason i mention all of this is because it may or may not have something to do with my body reacting differently to alcohol? it’s hard to say when it started but i want to say during that summer or beginning of sophomore year maybe? almost out of nowhere i couldn’t handle my liquor and went from being a heavy drinker to a lightweight. i’m talking one shot or beer made me feel ill and i got drunk easier. i also get nasty hangovers now. i could no longer keep it up. or down lol. i suspected a stomach ulcer but have no clue. i can drink a little more now because i stopped drinking so heavily and lately i’ve only been doing it on occasion but i would love to know why this is happening to me and if there is a way i can get ever get back to the way i used to be because it really bothers me.