Not 100% sure on this but humans have two ways to metabolise alcohol, with one that results in hangover and other where liver breaks alcohol in one go, meaning it doesn’t release internediate product back into blood, triggering hangover symptoms. Proportion between those two differs from human to human and with age.

Downside of being in „mostly second group” is high risk of liver scarification.

I also don't get headaches from hangovers, the only symptom I really get is nausea. A handful of times bad enough that I actually puked while hungover, whereas I've never thrown up while drunk.

This is me. I think I've only puked while literally binge drinking and my stomach was full. But I always puke the next day. No headaches, just nausea until about 4p then I'm ravenously hungry.

Stomach ache, tiredness, nausea, dehydration, ...