Some minor/hard-to-notice health-related things can dramatically reduce alcohol tolerance and/or give “hangovers” shortly after starting a session.

For me, inflammation is a big cause. I have (barely noticeable) cat allergies, and (obvious but hard to avoid) food intolerances & gut issues. If I don’t stay on top of avoiding triggers, my alcohol tolerance goes from multiple G&Ts giving a nice buzz, to 1-2 sips of G&T giving dizziness and headaches. Electrolyte imbalance can also cause it. I’ve found I have to add magnesium and potassium salt to my diet, or else I generally feel tired more, and my alcohol tolerance plummets. Once you start controlling these factors, you’ll start getting clear feedback from your body when you have too much or too little salt, in the form of water and food tasting different and general feelings of tension or tiredness.

My advice: try antihistamines, easily-digestible meals, and/or sports drinks for a few days before you drink. If those help your tolerance, you probably have some health stuff going on - figure it out and you’ll probably find a way to generally feel better.