I don't believe it's a headache or hangover. I think I've heard it in a chemistry context. However, it is made up of 2 words after + swell and my guess is that the meaning can be understood by adding the meanings of those two words. What was the context you found it in?
The text speaks of the days that followed a highly controversial public debate, so the use of the word «afterswell» in the context is figurative for sure. At first I had two possible candidates:
Both are translated into Spanish with the very same word: resaca
Anglika, do you mean the repetitive waves like when you throw a stone into a pool?
Not quite. When there has been a storm at sea, the after-effect of the storm is to produce very large swells in the water [which in a small context are ripples]. These swells will, when hitting land, become large waves.
In the context of a post-political debate, the afterswell will be the reactions in those listening to or reading about the debate = the way in which their opinions will grow or change in such a way as to change public thinking/opinion.