The 31-year-old defending champ lost control of her dragster right off the starting line -- veering across her opponent's lane and smashing into the far wall. Her car caught fire as she skidded back across the track and hit another wall.

Miraculously, rescue workers were able to pull Brittany from the car VERY quickly with no apparent major injuries.

She was raced to a nearby hospital and was evaluated overnight. Doctors say she suffered no major internal injuries -- only minor bruising to her lungs.

Brittany -- daughter of racing legend John Force -- has been shockingly smiley and happy through the ordeal ... even posting a thumbs up pic from the hospital!

"A little bit of a rough start to our season today, but I'm in good hands and will be checking out with only a few bumps and bruises."