Carrie Fisher -- Hospitalized After Bipolar Episode on Caribbean Cruise Ship

Carrie Fisher was hospitalized following a bizarre performance on board a Caribbean cruise ship last week -- and her rep tells TMZ, it was all caused by the actress' bipolar disorder.

Video footage surfaced of the performance -- which took place last week on board the Holland America Eurodam in the Caribbean -- showing Carrie slurring a couple off-key songs. Her dog even POOPS AND PEES on the stage while she's singing. It's pretty nuts.

There were reports Carrie -- who was featured on the cruise as a "surprise" celebrity guest -- was wasted at the time of the show ... but her rep says the on-stage breakdown was caused by Fisher's bipolar disorder.

The rep says, "There was a medical incident related to Carrie Fisher’s bipolar disorder. She went to the hospital briefly to adjust her medication and is feeling much better now."