One of the things about exercising is that I learn more about how my body works.
From past experience, I already knew that I feel crummy -- i.e. "nauseous" -- if I do a full-scale workout after being "away" for a while. For example, back in Seattle I took a semester off from kung fu (I had a class that conflicted): after the first kung fu class upon my return, I had to dash to the men's room and throw up. Yep.
So, I prevent getting sick from exercising by doing one or two partial-intensity workouts in the week leading up to it
I've now learned something else: if I'm working out and I drink diluted juice during the workout, instead of water, then afterwards I end up feeling crummy and throwing up. Although doing sit-ups as part of the workout possibly didn't help.
So, OK -- I get it. ;)