Log into Self Serve and click "Change Sim Card" - and confirm that the last four digits match the card you have in your phone... If it is different, your account was likely hacked and you have been sim-jacked, and need to get in touch with the moderators ASAP to get them to reinstate your existing SIM card. Also check for any fraudulent activity on banking, paypal etc that might use your number to send verification texts for login or password resets. PayPal has been a popular target recently.
Lastly for your own interest you can plug your email address into https://haveibeenpwned.com/ to see if your email address has shown up on any well known recent data breaches. Don't re-use passwords that might have been used on any services found with your email address....
