As per the press release by FaceBook due to a latest bug the post which you intended for your friends only have become public.
Facebook said this affected users posting between May 18 and May 27 as it was implementing a new way to share some items such as photos.
“Starting today we are letting everyone affected know and asking them to review any posts they made during that time,” Egan said.
“To be clear, this bug did not impact anything people had posted before — and they could still choose their audience just as they always have. We’d like to apologise for this mistake.”
So you should check setting of all the post you make between May 18 and May 27, 2018 and change their post settings.
It is also suggested NOT to frequently change you FaceBook Profile Photo and COVER photo as it always be public also do accept friend request from your known only.