Nicole Is Gonna Be Pissed

Nicole Richie is going to flip her when she hears what TMZ has:

While the cat's home nursing her celebuspawn, the mouse will play with his baby mama's frenemy!

Joel Madden and Lindsay Lohan got cozy at the Hornitos' Cinco de Mayo party last night. Interestingly, Joel's girlfriend Nicole Richie -- who is of Mexican descent -- was not at the event.

Someone better run for the border.