I recently (and stupidly) created a Windows SBS 2011 user whose password was the username. Multiple unauthorized people across the Internet used RDP to log in as that user. How did they find the username?
Your account may have been brute forced. This is when someone writes a script to (not manually, automatically) try all combinations of usernames/passwords up to a certain length.
For example, one alogrithm my start out with a then go through z then try
zz for usernames length up to a certain number. Some/Most brute-forcers also check for the most common usernames such as "admin", "root", and common names.
If your username was short, it's much more vulnerable to brute-forcing, since the longer the password is, the more possibilities there are to the username of that length.
With crystal ball in hand, I predict your username was either a) really short or b) a common name.