Why would someone do this?

A girl claims that she was in a relationship with my boyfriend while we were together. Her details don't add up and my boyfriend denies it. Even though to me it sounds silly I listened to her story and checked some of her details which didn't seem to add up. Why would a girl make this allegation upon entering a new relationship with her new boyfriend? Why would she do this..? Why does she include so many details and seem to have no regard for this story affects others?