Just going to put in my findings and what you should follow:
Steam has been compromised by either an employee or other. You can see another persons payment info by clicking Account Details. Do not do this. If you have the mobile authentication service, you can remove your payment information and you should do this IMMEDIATELY. If you do not have this service, you are pretty much screwed. Do not sign-in or out of steam, just don't touch steam, if you can do everything from the mobile service. Do not buy any games! Remove your payment information if possible but otherwise do not touch anything