Demi Lovato -- Rushed To ER

9:45 AM PT -- Demi's rep says she returned home shortly after her ER visit and is "feeling much better."

Demi Lovato was hospitalized Tuesday for a nasty lung infection ... TMZ has learned.

We're told a friend took Demi to the ER at Providence Tarzana Medical Center because she was having significant trouble breathing.

Doctors did a bunch of tests and decided her problem was flu-related. They gave her medication, including a Z-Pak.

As of late Tuesday afternoon, we're told she was still at the hospital.

The timing sucked, because Demi is recording an album and she had studio time Tuesday with a big producer and she had to cancel.