What are the websites that you visited recently? My suspect is you just became careless and you logged in to a phishing link though you said that you are careful of not giving out your keys, you are still vulnerable to log in to a fake site / phishing site that you aren't aware if you are in a hurry or you just really don't care of that site you are logging in. Like what has been said, you probably downloaded something that contains malware that affected your computer that allowed the intruders to touch your files on your pc.