I am now posting with my friend's account, I cannot sign in with my own!

Each time I tried to sign in, a web page opens and show the following:

You haven't signed in from this location recently

To help protect your account, we want to make sure this is you. How should we verify your account?

My alternate email address received a verification code and after I copied the code into the verification page, the process start over again! It asks me to verify my account and want me to give a code (NO error messages displayed).

I tried to verify by answering my security question, but after I give my answer, nothing happened, the web page just refreshes and asks me to verify again!

the most weird thing about this is: I can change my password by answering my security question or get a password reset link via my alternate email address (and a verification code as mentioned above)!

Please help me with this, thanks!