The England international scored for Dortmund against City in their Champions League quarter-final on Wednesday, with Guardiola's side eventually prevailing 4-2 on aggregate.
Asked about Bellingham's performances over the two legs," Guardiola replied: "I cannot believe it, maybe he's a liar!"
"He's so good for 17-years-old, he's a fantastic player.
"There was one moment when he didn't get the ball from central defenders, how he shouts and demands that ball to him at 17 means a lot.
"I spoke with his manager, Edin [Terzic], and he told me what you see in these two games is like every training session."