I've just been sent an email saying my sign-in ID has been changed successfully, but I never changed my sign-in ID. What do I do?

I've already messaged support of course but I am wondering if there is anything else I should do, preventative measures to protect my information etc.

Also, do I need to worry about Sony's support screwing me and not helping? Because I've heard some horror stories about people getting the run-around from them.

Thank you.