Lately I've been getting a lot of warnings about SSL certifications on my pc, Empathy keeps telling me that Facebook's certificate is self-signed and can't be trusted, and also, there are occasional warnings in Google-Chrome about security. I remember the last one saying that that the page is secured but some of the resources that the page is using are not from a secure connection, something like that.

Is my pc hacked / under attack? How can I check that, and if so, how can I safeguard myself?

PS: One thing that comes to my mind is that I might be under an arp poisoning / spoofing attack.