I bet some kind malware / browser addons / keylogger is on your PC
My friends almost lost all his BTC, but he's enabled 2FA
, i reccomended 2FA SMS Message
Expect, your phone is stolen
What browser and plugins are you using? Since Mandrik said it could be due to a third party plugin, knowing this could certainly help.
I'm using IE.
Though I am making the switch to Firefox.
Don't know of any plugins
Old IE version is easily hacked by hacker
Chrome / firefox is more safer