Paris Hilton may be back on the market! Nick Carter her boyfriend of about five months is telling friends he gave her the heave-ho over the Memorial Day weekend, according to a Carter pal. "She put that boy through the ringer," the friend says, adding that "Trust issues," stemming from Hilton's several rumored liaisons, did the couple in. Carter already has his sights set on Jessica Simpson's little sis, singer Ashlee Simpson, 19. Carter and Ashlee flirted heavily with each other at a Clear Channel radio gig in Dallas on April 29. You'd think Carter would be man enough to break the news to Hilton-her reps insists she and Carter are still in love.