Growth hormone spurts. Your body releases growth hormone in response to exercise... and sleep. GH makes you nauseous. What you are describing is you getting the GH to grow... unfortunately it makes you feel like puking.
Doh, because I workout first thing in the morning before work I don't ever get to eat before weekday workouts. I get up at 5:45 am to workout and I can't imagine getting up any earlier just to eat.
But I never feel nausea afterwards. If anything, I guess I just don't left quite as much as I would otherwise, so weekends are for PRs.