"It's just started," says the source. "But they're really cute together."
The two were first spotted getting cozy Friday night at Los Angeles's Mondrian Hotel, where they laughed and talked the night away, finally closing down the bar, says the source. The next day, TMZ's cameras caught them shopping at Nordstrom at The Grove and picking up refreshments at the Coffee Bean and Tea Leaf.
Phelps's reps could not be reached for comment, but the Gastineau source says the two bonded over their sports-related backgrounds. Phelps holds the Olympic record with 14 gold medals in swimming, while Gastineau is the daughter of gridiron great Mark Gastineau.
"She thinks he's a really sweet guy," says the source. "She understands his athletic background since her dad was an athlete."
But before anyone starts ringing wedding bells, the source warns that the romance is still brand new. "Brittny's been in long-term relationships her whole life. She's young and she's just having fun."