The 25-year-old gave birth five months ago to a daughter named Genevieve.
After giving birth, Upton has been working out regularly and thinks she's getting "stronger" despite what the scale says.
"It's kind of a buzzkill when you see that number [on the scale]," she told People magazine about losing her pregnancy weight.
"I am definitely making progress, and there's little wins. I personally don't love getting on the scale too much, because it fluctuates from so many things, like breastfeeding, or if I'm bloated or too hydrated or not hydrated enough," she added.
"It's kind of a buzzkill when you see that number. So I always try to focus on my weights -- am I going up in weights, do I feel stronger when I'm doing lunges. I'm definitely getting stronger and feeling better," Upton explained.
She insisted it's not so much about her bikini body as it is about her mental state.
"My whole mindset changed so much, instead of trying to fit into this perfect mold that I thought I was trying to be. As soon as I started caring about my strength, I felt healthier. I felt stronger. I was able to have more energy and be more active. And I was happier, honestly," Upton revealed.
The blonde beauty said her trainer has been easing her back into certain moves and she was startled by the fact that she lost so much strength.
"I felt very strong the whole pregnancy, and then you're starting completely over again. It all disappears! I did the workouts and I started at phase 1, which used to be so easy for me, and suddenly I was struggling so much," the model confessed.