Reba McEntire is thrilled her son Shelby has found his calling racing cars, but that doesn't negate the fact that she worries about him when he is zooming around the race track.

"That first time when that green flag drops, my stomach goes right to my throat, and then it goes back down and then I'm OK," Reba tells The Boot. "But, boy, that roar is tremendous."

While she assumed Shelby would follow in her footsteps and those of husband-manager, Narvel Blackstock, and Shelby's older brother, Brandon, and join the family music business, Reba was pleasantly surprised when he took up the sport of racing last year.

"This thing with Shelby and racing came out of nowhere," Reba explains. "Ever since he was a little bitty boy, we would be driving somewhere and he'd say, 'Mom, do you know what kind of car that is?' And I'd say, 'Blue?' And he'd say, 'No! That's a Mazda so-and-so-and-so.' He'd know the year and everything, and I thought, 'How does he know all that?' It's always been cars, and he was just so unhappy in college. I said, 'Shelby, what is it that makes you happy?' He said, 'Racing.' Well, you could have slapped me in the face, and I could not have been more shocked! I said, 'What do you mean racing?' He said, 'Mom, I want to race cars.'"

Shelby, who turned 21 in February, attended the University of Arizona last year, but quickly realized college was not the right path for him. A family friend's introduction to the owner of a racing team steered Shelby in the right direction.

"We were going to put him into the ITT Technical school and let him work on cars and be a mechanic, which I thought was ridiculous because he didn't know anything about being a mechanic," the songstress explains. "And then on his last day of school, a good friend of ours, Pat Manley, said, 'I've got a friend. I want you to go to brunch with me.' It was Jim Click, who owns car dealerships all over Arizona and had a racing team. They went to brunch, and Jim said, 'Hey! Do you want to go to upstate New York and work on my pit crew?' He said, 'Yeah,' and he did."

That job led to Shelby's latest. "This last weekend, Shelby was in the Jim Click car racing a Grand Am race," she boasts. "He went from 18 to third before he handed it over to Michael McGovern, who was the second and more professional driver of the team. Who would have thought it? If it wasn't for Pat Manley and Jim Click, Shelby probably would never have realized his dream."

Even though the racing world is different from what the proud mother is accustomed, it still encompasses a few familiar elements that have Reba feeling comfortable with Shelby's decision. "There's so many great people in the racing business, and I'm just thrilled to death," she exclaims. "It's like being in the rodeo business: you know everyone and it's all like a family. And then the music business: it's all like a family. I feel so grateful that he is so blessed to be in such a wonderful family again, but now in the racing business. So, I couldn't be happier for him."

Reba is taking most of the summer off the road, but you can catch her on the small screen being honored during the the Academy of Country Music's 'Girls' Night Out: Superstar Women of Country: Concert of the Year,' airing April 22 at 9:00 PM ET on CBS.