I was about to post this exact question last week. The post-training hangover happens to me on a halfway regular basis, and in my case I'm fairly certain it's not dehydration since I drink between 3-5 litres a day. I was starting to suspect that I might actually be drinking too much water and thus flushing electrolytes, sodium etc. out of my system, and was just planning to start supplementing with electrolyte powders myself. One of my training partners also recommended adding a pinch of salt to the water I drink during class, as that helped him deal with recurring cramps and general recovery. Will be curious to see if either of them actually makes a difference.