After three months of harmony, Katy Perry, 31, and her beau, Orlando Bloom, 39, have hit a sour note. At Coachella, “Katy caught him checking out a group of young, hot girls,” a source says of the stars, who attended the Indio, Calif., music festival the weekend of April 16. “Then, at one point, she noticed he was staring at someone they were hanging out with. Katy started yelling at him.” The “Wide Awake” singer has experience with flirty exes, including former boyfriend John Mayer and ex-husband Russell Brand, with whom she split in 2011. “Later, Katy told friends how the honeymoon period was over and she was going to take a step back and let Orlando prove himself. If he failed, she’d have no choice but to end it.…