Possible facebook hacker, please help?

For the past week, whenever I go on facebook I get this weird pop-up that says something about an untrustworthy certificate and that I might be connecting to a website that is pretending to be facebook, and that continuing could put my confidential information at risk. I don't know what to do! Usually if I click cancel a few times it goes away but lately it's been popping up a lot. I have tried clearing my cookies and restarting the computer 3 times! What should I do?