What did you set it to when you created (or updated) your account (have you got any other email addresses that you use) ? If you don't know what it is (and if you have one) then you could try the instructions half-way down that page and see if it shows :
- Navigate to appleid.apple.com using your web browser.
- Click "Manage your account"
- When prompted, sign in using your Apple ID and password.
- Click Password & Security
- You'll be asked to answer 2 of your 3 security questions before you can make any modifications. If you are unable to remember your answers, you can choose to send an email to your rescue email to reset your security questions.
Note: The option to send an email to reset your security questions and answers will not be available if a rescue email address is not provided. You will need to contact iTunes Store support in order to do so.