Miley Cyrus Emergency -- More Like MILEY SINUS EMERGENCY

Miley Cyrus' hospitalization was triggered by a sinus infection gone horribly wrong, TMZ has learned.

Reps for Miley tell us, the singer got an Rx for an antibiotic called cephalexin (brand name Keflex) to treat a sinus infection after getting diagnosed last week in North Carolina.

We're told within days Miley's throat swelled to the point she could barely talk. She was rushed to the hospital Tuesday night and, according to her rep, doctors determined she had an extreme allergic reaction to the drug.

We're told Miley never got a cephalexin script before so she had no idea she was allergic.

She'll remain hospitalized until her condition improves. Her doctor says an allergic reaction of this kind can last 27 days.