Username automatically changed?

I use WordPress as a CMS for one of my clients. I’m accessing the admin of the site quite often (about twice a week), using the same username and password. However, this morning I go to log in as per usual – my browser, Chrome, automatically fills in the Username and Password fields as it always does, then I get the message “ERROR: Invalid username.”. This has never happened before. I haven’t made any changes to profiles or passwords, and I doubt my client could either as they have only have Author privileges.

I prod further to find out what exactly is going on, and it appears my username which was ‘jason’, has been reset (or changed) to ‘admin’. I then try and login using the username ‘admin’, using each and every one of my passwords I have, but with each time my password is invalid. The only way I was able to remedy this was to reset the password for ‘admin’.

I can confim my username was in fact ‘jason’, as many of the pages are published by ‘jason’ and not ‘admin’! I also logged in using my client’s credentials and their account appears unaffected.

A little bit of history:
The site was first setup on my own server until the site was approved, I then migrated it to the client’s own server where it now lives. This was done over a month ago.

I’m wondering if it’s an issue with Chrome and caching, or something more serious like a bug or intrusion. All content on the site appears unaffected. Has anybody heard of this happening?