The temperature on the truck started rising toward the red zone, so I stopped and eventually the truck cooled down. What's wrong?
Assuming that the coolant level was okay....
The most common reason is mud blocking air flow through the radiator. Get your garden hose and adjust it to a wide spray and spray water back from the engine compartment side of the radiator. Don't go crazy with the water pressure since you may damage the core.
Another reason could be the thermostat. Get it fixed because it doesn't take much to warp the head and suffer head gasket problems.