04/10/2009 AT 03:20 PM EDT
Josh Hartnett has not let a few days in the hospital slow him down.
The Black Hawk Down
star, 30, who was hospitalized last week
in Los Angeles for what his representatives described as a gastrointestinal problem, appeared in fine fiddle Thursday night – at one point even taking the stage at the Florida Room in the Delano Hotel on Miami's party-rich South Beach.
"Josh jumped on stage with a local Miami band," says a source at the club. "He looked happy and was having fun."
Since leaving the hospital
last Friday, Hartnett has been having so much fun in fact that he hasn't been able to contain it to one coast.
Wednesday night, the actor hung out past midnight with several pals on the patio of West Hollywood's Chateau Marmont, where he has been a mainstay since mid-February.
It was from the Chateau that Hartnett was rushed to the emergency room on Monday March 30 complaining of serious stomach pains.
Hartnett's representatives did not return calls for comment. On April 3, his rep told PEOPLE the actor was feeling "fine" after being hospitalized.