Olympic swimming Adonis Michael Phelps — who won a record eight gold medals at the 2008 games — opens up in the August issue of Details that he ballooned, putting on 25 pounds, after his amazing feat. “It was weird going from the highest of the high, the biggest point of your life — winning eight gold medals — and then saying, ‘All right, where do I go from here?’ ” he says. “I wasn’t motivated. I did nothing, literally nothing, for a long time. I gained 25 pounds.” It’s hard to believe, but hunky Phelps’ friends even made comments about his weight. “A friend of mine and I were playing football on the beach in Miami,” the super-athlete says, “and somebody got a picture of us . . . [My friend said] ‘Bro, you gotta start working out, man. You are fat.’ ” Now slimmed-down, Phelps, who appears shirtless and muscular on the cover of the men’s fashion magazine, is getting ready to lead the US swim team in London in seven events. “I realized that I probably hadn’t reached my full potential,” he says of his comeback. “There was still more in the tank.” Phelps, 27, adds that as more young athletes plunge into the sport, his strategy has also changed: “At this point in my career, everybody has caught up. I’m fine-tuning the little things that add up to make a huge difference.” And despite previously packing on the pounds, he still allows himself to splurge. “I don’t count calories,” he says.