Marc Malkin

Lindsay Lohan is a working girl.

After a Hamptons getaway with Samantha Ronson, LiLo is on the New York City set of Ugly Betty, a source tells me, where she'll spend the next couple of days or so reprising her role as Kimmie, Betty's high school nemesis.

A rep for Ugly confirms Lohan's return and says she'll begin work tomorrow on the show's set in Queens.

She recently wrapped Labor Pains, a comedy in which she stars as a woman pretending to be pregnant so she won't get fired from her job.

But before returning to America Ferrera & Co., Linds and Ronson kept busy with their weekend in the Hamptons...

They arrived Friday night and shacked up at the Grey Goose manor estate and enjoyed an afternoon of shopping the next day in East Hampton. Among the goodies were a new messenger bag for Lohan and a Mabel weekender for Ronson from the Mulberry boutique.

Lohan also picked up some sweets at Dylan's Candy Bar, including "smoking reduction" candy pills.

After being chauffeured through town in a Cadillac Escalade, they returned to Grey Goose for a Nicky Hilton-hosted clambake. The festivities ended later on at the Singer22.com party at Lily Pond, where Ronson was DJ.

"Lindsay stayed with Sam all evening and danced by the DJ booth with Ronson's sisters [Charlotte and Annabelle]," a source reports.

A friend of Lohan's tells me, "I haven't seen Lindsay this happy in years."

Keep it up, LiLo!