Greetings,
While it sounds like your account has been accessed by someone, I sincerely doubt it was us. We only sign into accounts to correct issues reported by the customer, and even that is rare. For most problems we have tools that don't even require us to log in on your account.
If someone has been accessing your account then you'll probably want to call us so that we can confirm information and secure the account. Besides other steps, we can change the security answer which is probably what the other person used to access your characters. Please let me know if you have any questions.