Assuming so, it would have been good for the system to notify me immediately, or better yet, reject the user name before creation (assuming it is an automated check).
But no worries, now to figure out an updated user name.
There's actually a long list of reasons why it doesn't let you know right away. Some of them secret, most of them to do with spam control. One of the reasons is that people don't often feel bad when they get a note telling them they did something wrong. We want them to get the feel for the place first, then they can decide if they like us enough to change their name.