Why so depressed the day after alcohol?

Not drinking every month. But sometimes party happen, and we drink. The day after I’m so sad. It went worse after I reduced antidepressants. Im on the lowest dose now, and I’ve been using 100mg Sertraline for 3 yrs. Almost done with them, just few 25mg left. I went to a party last night and this day has been shit. On the antidepressant I didn’t feel that I’ve been drinking, not much.. but so sudden when lower dose I can drink like 1 Gin Tonic and the day after is shit..