When you say username, do you mean your email? If so did you change your email at any point, it may of reverted back to an old email and that's now your username... it keeps doing this to me :\
I do mean email, however its an account that I only made a few weeks ago and the email hasn't changed. I've just reset the password and it still refuses to log in.
Ah, asked because I have a username and an email I can log in with, which keeps changing :\ Hmm don't know what to say then really which really isn't helpful
Could shoot them an email maybe, found them helpful in the cases I needed them.