The evaluation of other species (small mammals, birds, reptiles etc) can be difficult, and sometimes what may seem like a small change can indicate a serious illness in such a small creature.
Generally speaking, any change in basic behaviour such as increased breathing effort, loss of appetite or lethargy and decreased movement should warrant a call to the emergency service to seek the advice of a professional veterinary team.
When in doubt it is always ‘better to be safe than sorry”. Many conditions are successfully treated when caught early.