BRITNEY SPEARS stormed out of a Malibu supermarket in a fury, say sources, after a bizarre encounter of the worst kind with a strange-looking, unkempt woman with a facial tic who approached her and said in a weird monotone: “Your husband and I… we had an affair!” Dumbstruck, Britney stood rooted in shock as the unattractive, obviously wacko babe — who never identified herself — babbled disconnectedly about “sleeping with KEVIN FEDERLINE for months… but now I’ve stopped seeing him… but I thought you had the right to know the truth… and you better check yourself…” Rolling her eyes in exasperation, Britney turned and ran from the store, say sources, looking VERY pissed! In her car, she immediately whipped out her cell phone and called… Kevin?… her doctor?… her lawyer?… the loony bin? Stay tuned.