Seems like some more info is needed here to clarify the situation. What was the post?
At face value, you have a private FB which you choose who can see and access. You posted or reposted something that was not acceptable for the general public (so it seems). Not an issue since it’s private, HOWEVER, someone you allowed access into your private space got this info and either shared it or altered it slightly and shared it. Thus making it public.
Seems aside from being more careful screening things when reposting, you should also re-evaluate who you trust and allow into your private spaces.