Feeling Icky the morning after a work out

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Ok, I need some advice. I started REALLY working out again this week with a friend. Monday I did C25K W4D1 and then we did a circuit on uppper body with some ab work. I know that I didn't have a good snack before that work out. I felt not great that night. Tuesday I went home and took the dog for a walk, again without a good snack. I woke up yesterday morning with an upset tummy and after some mint tea and yogurt I was ok to have a breakfast sandwich with some cheese and a hard boiled egg.

Last night I did C25K W4D2 and we did an upper body circuit. I did have some beef jerky and a granola bar about 2-2.5 hrs prior to the work out. But I didn't get to eat for a while afterwords, which was about 5 hrs from the afternoon snack. This morning again, not really nauseated but not feeling well. Now that I am at work I feel a bit better.

I don't know if it's poor nutrition, not enough water after the work out (I drink quite a bit during the day prior to a work out), not enough sleep. But I would gladly welcome any advice!