Jessie James Decker is feeling stronger than ever after three kids.

Just six months after giving birth to baby Forrest in April, she’s already lost 17 lbs. and is focused on getting back to her pre-baby shape.

“I was in the best physical shape of my life after my second child, and it felt so good, so I couldn’t wait to get back there because I felt so healthy and felt so active,” Decker, 30, tells PEOPLE during the photoshoot for her new campaign with the South Beach Diet.

The country singer had signed on to the South Beach Diet after giving birth to her second child, son Bubby, in Sept. 2015, and loved how the protein-heavy diet, along with an intensive workout schedule, transformed her body.

“Before, genetically, I could just do whatever and be thin,” she admits. “But after Bubs I had to work really hard, and I think after seeing these results I noticed muscles I never had before. I was super toned and I felt super fit and I just loved it. It changed my mental mindset, because it was all about feeling fit and feeling good.”

After seeing what her body could do, Decker was ready to tackle that plan again after Forrest arrived. But she didn’t put any pressure on herself to drop the baby weight by a certain date.

“I don’t ever worry about what anyone else says,” she says. “I care about my family and my own mental health and physical health. It takes nine months to grow a baby and I believe it takes nine months and a little bit more to get back to where you were. So I’m not stressing about it and I feel really great about where I am now.”

At this point, Decker feels like she’s made tons of progress, but she has bigger goals in mind.

“I don’t think I’m 100 percent there yet,” she says. “I definitely think that I have a little bit of a ways to go, but I do feel really good now.”

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Along with eating protein- and vegetable-packed meals, Decker works out whenever she can pass the kids off to her husband, the newly-retired NFL player Eric Decker.

“I’m weightlifting and it’s really fun, because I started at 5 lbs., and then I got up to the 10’s, and now I’m at the point where I’m using my 20-lb. dumbbells and I feel good. It’s a slow process but I can just see the gradual progress, and it feels great,” she says. “I’m working out even harder now.”

And though she’s lost 17 lbs., Decker says she now cares more about her muscle tone than the number on the scale.

“I’m not a big scale person. I’m big on how I look and how I feel, and I’m all about building muscle and toning up,” she says. “I used to obsess over that stuff back in the day and I just don’t anymore. I think how do I look, how do I feel, how do my clothes fit me, and I’m all about getting in the gym. I love lifting weights and I love being active and I look and feel better from that.”