I think heating them a little too much causes gasses to expand in the belly/digestive track. You have to find a balance between complete thawing/warming...and not warming too much. Try some different thawing/heating methods. I've found that thawing overnight in the fridge is a safe way to be sure the rodent is thawed completely through...then the warming up process doesn't take nearly as long, so there's less risk of gasses building up.