Samantha Ronson starbucks sunglasses lindsay lohan car shots coffee stripes

While Lindsay Lohan is busy celebrating her 25th birthday (and her recent release from house arrest!) ex-GF Samantha Ronson is all about spending time with her new ladylove.

The celebrity DJ hit up a local Starbucks with her new girl Erin Foster to get her caffeine fix, before hopping in her brand new Porsche and speeding off.

Okay, so we're not sure if these two are really dating - a source tells us they are. And just who is Erin Foster? Well if the cheekbones and jawline don't give it away, she's David Foster's daughter, sister of Sara Foster, who played Annalynne McCord's sis on 90210. And Erin's an actress as well -- her most notable role being that of Marissa Cooper's [Mischa Barton] nemesis on The O.C. way back when.

A source tells X17online Lohan is "heartbroken" over her ex's new girlfriend and that she's "madly jealous" of Erin.

Lindsay and Sam split in 2009, but the Mean Girls star never seemed to have fully gotten over ex -- she even moved right next door to the DJ's Venice home.

According to our source, Sam has dating Erin, 27, for the past few months, "flaunting" her around Malibu, where Lindsay always hangs for the 4th of July weekend. We're told Sam is fed up with Lindsay and is doing this to make her jealous.