Treated for Carbon Monoxide Poisoning

... During Race

This is scary stuff ... NASCAR driver Denny Hamlin fell ill while behind the wheel of his #11 car during a race at Dover on Monday when carbon monoxide gas began seeping inside of his ride.

The 38-year-old says he felt sick and experienced double vision and was treated by medical staff on pit road after he finished the race in 21st place.

.@dennyhamlin was being attended to by medical staff on pit road once he exited his car. Looked exhausted.

He was seen on a golf cart being wheeled away for further treatment. #NASCAR pic.twitter.com/2pEIA6Ac1s

— Davey Segal (@DaveyCenter) May 6, 2019According to the AP, Hamlin's car had been struck by debris on the track -- which knocked out…