But the singer got sick during her trip from Scotland to Los Angeles and had to cancel her second major TV appearance.
"Her doctor advised her not to sing due to a severe throat infection," her rep says in a statement to PEOPLE. "Susan is very upset at letting her fans down. Their continued support means so much to her."
She also had been set to sing Lou Reed's "Perfect Day" on America's Got Talent recently, but the spot was scotched because of an issue over the rights to the song.