The `Mysterious Girl` singer was jogging near his home in East Grinstead, West Sussex, England, at 5.30am when he suddenly collapsed in agony.
A friend told The Sun newspaper: "Pete was doing his usual morning run round the private estate where he lives when he felt a crippling pain. He screamed out in agony and crumpled to the ground clutching his chest. He thought he was having a heart attack."
The 37-year-old pop star - who has two children, Junior, five, and three-year-old Princess Tiaamii, with ex-wife Katie Price - was helped back to his house by a passerby, where his brother Mike immediately called an ambulance.
Upon arrival, paramedics found the Australian singer had injured muscles in his chest.
The pal added: "It`s not serious but the pain is immense."
Peter recently revealed he is undergoing a gruelling workout regime as he prepares for a forthcoming mystery venture.
He said: "I`m working out like a demon at the moment because I`ve got a very exciting secret project coming up which means I need to be in the best shape of my life. I`m working my backside off every day!"