My Ex[20/f] from high school said I [20/m] raped her.(x-post from /r/askreddit)

Back story; Me and my ex dated for a year went through a nasty break up and at a party told her current boyfriend that i raped her to start a fight with me. I did not rape her for starters told my current girlfriend about this and my current girlfriend does not know what to think about this because she is actually a rape victim and it is a very sensitive thing. She can not take my word for it because her attacker tried to do the same road as "I didn't do it, I swear". She has a hard time believing me when I say it. So Reddit I come to asking what should i do and what can i do. I really do not want this to ruin my relationship.