How did I go from a heavy drinker in my late teens/early 20s to absurdly alcohol intolerant after two years of drinking very little?

[Note: Am 26] I really don’t get it. I know alcohol tolerance goes down to an extent as you age, but since reducing my drinking to almost zilch, I’m insanely sensitive. Like I’m literally sitting here very possibly nursing a hangover from one G&T last night. A couple months ago I had four drinks in an evening with plenty of food (which I hadn’t done in nearly two years) and literally woke up wanting to die the next morning. A couple beers and a glass of wine in 2024 had me vomiting and bedridden the next day until 1pm. Ditto for a single bottle of apple cider around the same time.

To make it weirder, it’s not entirely consistent. One standard of whiskey doesn’t seem to do this, and neither does a glass of good quality white wine. And somehow, one pint of Guinness doesn’t punish me yet, unlike every other beer I’ve tried in this time. But if it’s an allergy, I cannot fathom what my problematic drinks have in common that my “safe” (so far) drinks don’t. It doesn’t make any fucking sense.

(I don’t get any of the characteristic symptoms of histamine intolerance, so it’s not that at least, and nobody else in my family experiences this – my very moderate 60 year old parents could drink me under the table until I die at this point.)