His first clue was the horrible smell.
His next clue was a mouthful of foreign-tasting hair that accompanied his first morning-grooming lick.
Instead of the familiar arched-back, hair-on-end reaction to danger, he felt his snout curling menacingly and heard a threatening growl roiling up from within.
Hairball, always the optimist, hoped that he was suffering some type of delusion or hallucination. After all, that rat that he finished off the night before had tasted a little odd.
His next reaction was to attempt to retreat to his favorite hiding place—under the bed, but his retreat was blocked by his now-too-large rear end, which lodged firmly under the bed frame. He escaped only thanks to a forceful sneeze, induced by a large ball of cat hair (his own), that he was now extremely allergic to.
As Hairball catalogued all of his former sins, searching for an explanation for his predicament, it occurred to him that he had become, in very literal terms, his own worst enemy.