Demi Lovato has been basking in the success of her hit single “Cool for the Summer” and during the first day of iHeartRadio Festival in Las Vegas, the singer hit the stage in Denim Shorts and fishnet to perform the sultry song, along with “Really Don’t Care” and a surprise show-stopping duet with Third Eye Blind frontman Stephan Jenkins on the band’s ‘90s hit “Jumper.”