It sounds like your account was compromised.

I don't know your browsing habits, if you check porn sites, click those emails from those generous Nigerian princes, buy from RMT, use anti-malware programs or browser addons such as Ad-Block (& Ad-Block Pro) and No Script (even legit sites can have an ad that has malware/keyloggers attached to it and the site-owner might not even know), but those can decrease your chances of any of your online accounts being compromised.