Yep, a quick change in diet (from grain and hay to grass) can cause stomach upset. A heavy dose of wormer on a heavy worm load can also cause stomach ickies. Generally you start with 10 minuet turnout, slowly working your way up to an hour and longer.
He could also just be having an icky day, ate a plant that he wasn't supposed to. I'd watch for signs of lethargy, not wanting to eat, pale gums, stumbling gait, wanting to roll or getting up and down. I'd also check for signs of a fever or high temp (a quick way to check is to lay your hand on the inside of the back leg, if it feels hot or is sweaty, not a good sign).
Whats his weight? When was he last wormed?
Wait! I'll fix it....