Minecraft – Changed Minecraft account name & now it says I’m not who I am

There is no way of changing that. Users pick what they want that to say.

In the "add server" menu, users can put a name in. (I usually just put a space, so that all i see it the MOTD)

The MOTD is all that you can change yourself for all people.

It is not possible to get a username like that. Mojang are trying their best on the censorship of usernames. The most likely answer is that someone did something like /nick (your username) (the new username) as a troll or something.

I would recommend going into a single player world and typing something in chat. If it is the same then contact Mojang support immideatly and change your password to a really complex one.

EDIT: Contact Mojang support now. This here shows you that someone has changed your name and when. Also when you can next change your name back manually. You have to act now for your security's sake.