Sand would be in the intestines, which are usually full of digestive "things" so if you can hear a swirl your horse probably has nothing inside his tummy. Sand issues are more common on the costal states then inland. Pysllum Husks will clean out sand along with anything else inside the tummy. I think I would use that first, before altering his whole meal plan, when he's already got an upset tummy. My bet is on a mild case of gas. If its gone on for more then a day or so I would be VERY concerned, because most tummy upsets lead to colic. I would have the vet out tubing my horse asap, but that's just me. I used to feed a horse who got mineral oil with his food to help him pass more easily.