Its your liver needing a rest and time to recuperate.
I used to get quite bad hangovers then I did 6 months in a dry country and stopped having them.
The feeling crap after one I think is more down to fatigue issues. Even one beer effects your sleep and anything which knocks you back even slightly if your at the edge of fatigue makes you feel it 10 worse than when your at normal levels of rest.
I know folk that swear by liver salts making a difference along with a huge dose of cod liver oil everyday.