Shopping at a health store, 20-year-old Christina Aguilera nearly fainted when Brad Pitt strolled in! (Ohmigod, OHMIGAWD!) She rushed up, introduced herself — then mumbled that she LOVES his films . . . he is SO cute . . . and (ohmigawd!) they share the SAME BIRTHDAY, December 18! As Christina chattered, Brad got twitchy. And when she asked what he was doing on his birthday, he sputtered, “I’m spending it with my wife . . . nice meeting you . . . !” — and hightailed outta there!