If it was right after he ate a large meal he might be dealing with an overful stomach. Chams have small stomachs and no sphyncter between it and the esophagus. If their stomach is full or they drink too much water, once they start climbing around they can aspirate stomach contents back into their throat. I don't think anything is actually wrong. We tend to feed our chams very large insects compared to what they would get in the wild.