The mystery illness that caused Janet Jackson to cancel several dates on her Rock Witchu tour has been discovered: vertigo-inducing migraines! According to a statement from her rep, Janet was suffering from vestibular migraines which cause the feeling of vertigo. But the star is receiving treatment and ready to go back onstage. "[Janet is] a world-class entertainer and needs to be at the top of her game to give her fans the show they expect," her manager, Kenneth Crear, said. "She's feeling much better and is ready to hit the road again." Her tour is scheduled to resume tonight at the Verizon Center in Washington, D.C.