Number at end of username suddenly changed

When I first joined this site a couple of days ago, my username was Shona0, I think this was still the same when I logged back in earlier today, but when I logged in about half an hour ago, I noticed it had changed to Shona1. I didn't do this (not sure if it's possible to change usernames and don't know how anyway). Is this a glitch or something? Just curious.