It's sad but animals do die and unless you kept the body in the fridge and took it to a vet shortly after for a necropsy (which I would have done since there was no real quarantine) there is really no way to know what happened.
Many people like to jump the gun at the slightest little symptom that may resemble IBD however IBD is rare, and drawing conclusions that one may have IBD in their collection without proof is not something that should be done here.
Did you treat for mites upon arrival?
Do you have a thermostat to regulate the temps?
Was you house sprayed for bugs?