I am so sorry Micha is so uncomfortable......poor baby.
He may have caught his harness on something and sprained or bruised himself.
Feel all over the area the harness affects, ribs,back, belly, legs, underarms, etc for any pain response.
Also check his body for any scratch,bite,wound or chaffing.
It is also possible a human creep hurt him.
I would also look in his mouth for a stuck item or any abnormalities.
If you find a sore spot on his body apply a cold compress to the area 5 minutes on and 10 off for an hour. Then switch to a warm compress same intervals.
Continue with the one that gives him the most relief.
Offer him canned cat food or tuna and let me know if he eats or not and how he is doing. A cat whos in pain will usually refuse food so his appetite is the best barometer of his health.
I hope he feels better very soon.
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