Because oxygen levels in your blood have increased significantly and the brain, accustomed to lower oxygen delivery, is adjusting to a richer supply than it has received in years.

Because smoking was suppressing the immune activity in your mouth, and after quitting, the immune system rebounds, temporarily overshooting in ways that produce ulcers as a side effect of restoration.

Chest tightness and discomfort are common in early cessation and are usually benign, but chest pain after quitting requires more careful reading than most other withdrawal symptoms, because the causes range from entirely harmless to genuinely urgent.

Because nicotine was regulating your digestive system, and without it, the gut is recalibrating in ways that produce nausea as a predictable, temporary side effect of restoration.