The pair were spotted kissing as they watched Florence and the Machine at the festival's Bing Bar venue.
A source said: "They were making out in the corner of the dance floor upstairs."
Another witness said that they were "laughing" and looked "totally smitten" and a third spy told People magazine they were enjoyed a meal at 350 Main Brasserie, where they were "giggling throughout the entire dinner".
Chelsea was rumoured to be dating 50 Cent and the speculation started late last year when they were spotted on a date in a jazz bar in New Orleans, after attending a birthday party for one of Chelsea's friends in California.
However, the funnywoman denied rumours of a romance via her twitter page, saying their meetings were strictly for business purposes.
She said: "Everyone, calm down. I met with Mr. Cent about a potential project. There's nothing to report yet, I'll let you know if there is."