If this was directly from the minecraft official website where you type in your new user, It may be that someone already has that username. But then again someone did use the same name as Malkamk. If it was on the server whilst you where connecting then It may be because it is or was still processing. Or, if it was off the forums user, maybe you misspelled something.
Instead of simply re-opening Minecraft, log out and back into the launcher again with your new username.
