Why has my alcohol tolerance decreased a lot recently?

I know this isn’t a sub to ask for medical questions but idk where else to post this and wanted to ask if someone has made similar experiences.

Like 6 months ago or so i could drink 0,1L of vodka (37,5%) in one sip and i’d feel slightly drunk/ tipsy but would still be fine. I’m F19 and neither my weight or my height has changed during this time. I also take antidepressants (Lexapro) but i’ve been taking it for years and have been feeling just fine when drinking.

I do drink regulary, like 3-4 times a week but never to the point of blacking out. Recently (in the past 2 months or so) i started feeling nauseous and getting headaches after having just a few sips of vodka very often. Instead of feeling euphoria i start feeling dizzy and tired.

I don’t know where this is coming from, i know i probably drink too often but i thought my tolerance would increase instead of decrease.

Has anyone else experienced this and maybe knows why it happens?