admin name has suddenly been changed! Should I be concerned?

Tried to log in as per usual and WP wouldnt let me. Tried to reset my password and noticed in the email that my user name had been changed to rOOt – this is slightly worrying as I’m not sure if I’ve been hacked? Why would that happen otherwise? Also now I can’t change it back to my old username because WP doent let you edit usernames. Can someone please help me out here