It was probably someone in a group if you posted in the Geneva/St.Charles groups. I doubt someone on your friends list cares. That’s unfortunate because it doesn’t really matter and Facebook wouldn’t have made you change it if someone hadn’t said something. :\
Do you know for sure that it was a person? They do have an algorithm that looks for fake names. It causes a ton of issues for first nations and aboriginal people; up here there lots of people with last names like Chocolate and Football and other things that FB thinks are made up. 🙄 That said, some people also have good reason not to use their real name, so I don’t think it’s fair.