According to the AP, Hamlin's car had been struck by debris on the track -- which knocked out his rear right crush panel and created an opening which allowed the dangerous fumes to enter the inside of the car.
Video from reporter Davey Segal shows Hamlin receiving oxygen and water. He looks exhausted -- but overall okay.
FYI, carbon monoxide poisoning is no joke -- it can be deadly if inhaled at high levels.
The good news ... Hamlin told reporters he felt much better on Tuesday and expects to race at Kansas Speedway this upcoming weekend.