You'd be surprised how often you can feel like you're in good terms with someone only to find out they hate your guts for no apparent reason.
Think about it this way:
You talked to your ex-girlfriend, who you said is unattractive, about your recent conquests and she felt threatened, intimidated or whatever and decided to make up some story saying "oh, I'm getting tons of nookie too you know..." but she was stupid enough to make it sound completely bogus. If the story was true then why would she feel offended about you calling her on her lie? She'd just say "whatever", after all she would still be getting her nookie and feel superior.
Learn from this situation and never discuss girls with someone you've dated before, specially if that person is single and/or needy.