A bad tooth perhaps.
Forgot to brush this morning?
Because it's a cat!!!!
Bad tooth, bad diet, or a GI issue.
They lick their butts! Perhaps it is what they lick! LOL!
Most likely a dental problem. They can develop some fairly severe dental issues without any outward sign. My car began having bad breath. Trip to the vet showed massive plaque build-up and 4 teeth were so rotten they had to come out, Breath was much better afterward, and I am sure the cat felt better. A friend failed to notice the problem, and by the time it was really noticeable the cat couldn't overcome the infection and died
Severe bad breath can be caused from a few things .. But primarily it is either dental or digestive issues. A vet could determine which it is .. In either case your cat is not healthy and could use some medical care