<<Disclaimer: Verify this information before applying it to your situation.>>

Thank you for your responses.  The consensus is that it is highly
improbable that heating food in a microwave would be a source of
contamination, even if it were pretty dirty.  (It wasn't ... well not
horribly dirty).

I ran an experiment ... heated the same food (rice, water-packed tuna,
velveeta cheese slice) in a brand-new, never used microwave ... same
hideous reaction within 15 minutes of eating it.  That, for sure, rules
out the microwave as the culprit.

Interestingly, there was the summary on Kraft cheese slices on the list
at the same time.  I rechecked my Canadian Celiac Association book
regarding Velveeta cheese slices, and they ARE listed as acceptable.
There is nothing on the ingredient list on the package that would cause
me to suspect the cheese.  So I'll try just rice and tuna today and see
what happens (sounds pretty dry and bland, eh).