As Natisha said, it could be something important or just a passing thing. I wouldn't worry unless it were to continue. Our horses get loose stools once in a while, but it usually is back to normal by the next bowel movement or within 24 hours.....
If it continues for more than a day, I would make sure the horse has water at all times, and a salt/mineral lick and give the vet a call.
Also, you may want to consider either feeding your horse a daily probiotic/prebiotic supplement or buying some probiotic horse cookies to keep on hand for these bouts of diarrhea and for after worming. Worming can cause transient diarrhea in horses, our vet said.... we give ours a daily pro/prebiotic so they usually don't get diarrhea....not REAL diarrhea...what they sometimes get is loose stools, but not watery.
I wouldn't worry unless the stools are consistently watery and/or if it doesnt pass within a day or so....oh, and to answer the question, stress could cause bouts of diarrhea, I would think.
Last edited by Beauseant; 12-01-2011 at 12:29 PM.