Any time you suspect something like this you should immediately change your password. The "user name" you are seeing is your forum name that you can set in your COD account/profile. That is also the name that will appear in clan chat. You can change that here: https://profile.callofduty.com/cod/login
A support case was created for you and you should be contacted shortly by a Customer Support Representative, if you haven't been already. You can view and update your case at the Activision Support website. After logging in, go to "My Support" then "My Cases".