You might be heating them too hot. Sometimes if you heat them too hot they "explode", but in those cases the organs are usually very hot. Like burn-potential hot.
It also happens with rats/mice that are going kind of bad sometimes. Does it spell spoiled/rotting to you?
How are you thawing your rodents?
Honestly bathing him might have stressed him out more. Anyone else have input on that?
Re-reading your post, it sounds like the rodent might be decaying.
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