Nicole Richie Catches Joel with Lindsay Lohan!

While Nicole Richie was home caring for her baby, her fiancé Joel Madden spent a boozy night partying with Lindsay Lohan. Needless to say, Nicole was crushed.

The trouble started for the new mom after she decided to stay home with their 4-month-old daughter, Harlow, while Joel deejayed at Hornitos' Cinco de Mayo party at Crown Bar in L.A. However, while there, the Good Charlotte rocker seemed more interested in a seductive Lindsay than in spinning records.

"Their heads were practically touching, and he had his leg over hers," says one onlooker. "Joel has a flirtatious side, and he definitely wasn't turning her away."

Nicole was immediately alerted to the situation by her arch-frenemy Paris Hilton, who sent her a devastating text message: Lindsay was all over Joel!

Making the situation worse, Nicole frantically tried to call Joel, but he wasn't answering his phone.

"He was so zeroed in on Lindsay, he never heard his cell ring," a friend of the couple tells Star. "Nicole felt so incredibly helpless and upset that she threw her phone against the wall in a rage."

Was Lindsay making a play for Joel? What happened when Joel got home? How does Nicole feel about Lindsay? Has Joel & Nicole's relationship lost it's sizzle? Get all the details in the new issue of Star, which is on sale now.