You need to go to the vet and get a portion of Bute. It sounds like your horse had a episode of Colic. You Probably got lucky this last time and he worked his way through it. Horses WILL eat themselves to the point of no return, especially grain. They really need a steady diet of hay all the time, plenty of water and small portions of grain and really if they are eating lots of hay you can get away with no grain.
If your horse starts to bite his side, flick his tail, have runny poop, roll and NOT eat, it is colic. They will sweat in extreme cases too. Listen for gut sounds, this is important.
If your horse starts to bite his side, flick his tail, have runny poop, roll and NOT eat, it is colic. They will sweat in extreme cases too. Listen for gut sounds, this is important.