"I looked pregnant," he tells Entertainment Tonight in an interview scheduled to air Thursday night. "It was upsetting. It's [usually] easy for me to brush stuff off of my shoulder. [But] when I see a pictures [of me] like that, and people are calling me 'K- Fat,' I had to do something about it."
One reason for the weight gain? The former dancer says he had stopped exercising and started eating too much. At his heaviest, he says he tipped the scales at 240 lbs.
"I loved to eat pizza and pasta," Federline, 31, says, "so between not working out, and turning 30 years old, [the weight] started coming on real fast. I had my own mother telling me she’s worried about my health."
Plus, his weight gain adversely impacted his relationship with sons Preston, 4, and Jayden, 3.
"I couldn't run around with my children as much," he says, "and was being lazy and over-eating. I've always been a fit guy, so those were wake-up calls for me."
That's why Federline signed up for VH1's Celebrity Fit Club 7: Boot Camp, which premieres Feb. 8, where he was joined by his ex Shar Jackson, the mother of his kids Kori and Kaleb. "We're doing this for our children," he says. "We're really good friends for the sake of our kids."
As for his relationship with ex-wife Britney Spears, Federline says things are improving.
"Me and Brit are good," he says. "I don't talk about those things anymore. It's in the past. We've come a long way and I'm happy where we are now. Everyone knows we had a tough road.”