If it was a few days after feeding I'd suspect something with the food. How old are your rodents? How many grams are they (on average) and what is the weight of the snake? How do you thaw them out, is it possible it was still cold on the inside of the rodent? It could be handling too soon as well, I have a few snakes that are really sensitive to being handled even days after feeding, I typically wait five days now, maybe just handle a day or two before the next feeding.