You have probably installed a stealth program in your sytem. Those are secured sites and will not allow you access to them until they can confirm who and what and where you are.
This could have been installed by some third party update and you not be aware of it or if you have multiple users this program can be installed on their settings but still affect you too. This same thing can happen if you try to hide behind a proxy server too as the secured sites must be able to link directly back to your computer for verification.
