When entering a shed cycle, sometimes they can wheeze due to the loosening skin around the nostrils.

Of course, a retic that was compromised and stressed due to mites and probably insufficient temps etc are more susceptible to infection.

I make sure the cage is warm thru and thru, no cold spots, though you can still have a gradient, like low of 80. Make sure it is hydrated - may need a soak. Sometimes, that plus food makes all things better. If not, a vet can help with meds. Too cold a cage will make things worse no matter what.