Welcome to SR! I was a binge drinker in teens and early twenties and definitely heading on the road to alcoholism, fortunately I found pot and got fully addicted to that instead! (Sorry, gallows humor!) But I found after I stopped drinking heavily that I also began to have a lowered tolerance.
For quite a few years after, my drink would literally be about an ounce of beer and that would give me a headache and hangover. I also used to get extreme hangovers when I drank more heavily.
I've joked that I used up all my drinking in my early years but haven't ever really found out why my tolerance dropped so drastically. Maybe I had extreme sensitivity and was an alcoholic or maybe my liver got overloaded, I don't know. But you're definitely not the only one!