Simpson finished singing "Boyfriend" on live TV Thursday, then told the audience she felt sick, Us Weekly reported.
She collapsed in an elevator moments later and was transported to a hospital by ambulance.
A source told Us Weekly doctors speculate the 21-year-old singer was suffering from exhaustion. Family members were waiting for an official diagnosis before deciding whether to go to Japan, the magazine said.
Simpson has been on the road for the past two months promoting her latest album, "I Am Me."
Her scheduled performance at Monday's Radio Music Awards in Las Vegas has been canceled.
Susan Sarandon 'very excited' about daughter's pregnancy
Scarlett Johansson steps out with fiance after pregnancy reveal