Well...is the EX best friend the same girl that she accused you of flirting with?
It sounds like you were flirting with her when your gf accused you...you were like "I don't know if I was flirting or not" - not that that's wrong...but maybe you liked this girl even when you were still with your gf? So....since you're not with your gf anymore I guess she can't really say anything...but I could understand your ex gf being a little pissed since she suspected it anyway! My advice to you is to always be honest with yourself...if you liked her before...you should have broken up with your gf earlier...but hopefully you are happy now...I dunno - just my 2cents - Peace!!