If nose, could be choke -- something stuck in throat, he's not drinking -- maybe can't.
Sand colic CAN cause diarrhea -- don't know where you live but if fed from ground, OR pastures very short, dirt/sand can build up in gut. BAD but can be taken care of with Sand Clear -- available TSC or most feed stores. Beet pulp fed fairly regularly can also prevent more of these issues. It helps attach to & remove much sand.
EITHER choke &/or colic is life threatening. You need a vet.
Thanks for the help and support, I owe you guys! Casper got into some weeds yesterday on the trail ride, we need to keep an eye out for signs of colic. If I can help you guys with advice in a thread, give me the link, or anything else I can do...