Why does Montag vomit at the smell of kerosene?

Montag vomited at the smell of kerosene because it reminded him of the old woman they'd burned that day.

"Yes, the bunch, the bunch, the bunch." He pressed at the pain in his eyes and suddenly the odour of kerosene made him vomit. Mildred came in, humming. She was surprised. "Why'd you do that?" He looked with dismay at the floor. "We burned an old woman with her books."