Guinea pigs sneezing?

I've had my two guinea pigs for about a week now. They have carefresh ULTRA (the white, more expensive kind), timothy hay, pellets, tunnel, pet bed, fleece blanket and a wooden get away. For some reason they've been sneezing, the little one more than the bigger one. They are both happy, talkative, popcorning pigs. They act 100% healthy, except for the sneezing. There is no discharge. Could they be allergic to something? Is the bedding too dusty? I thought carefresh ultra was the best of the best, or so I've heard. I'm going to the store tomorrow to stock up on couple things for them, including a new bag of bedding. Should I try another kind? Thank you!