Have you checked for viruses and spyware? I get these kind of notices sometimes when I'm using VPN's because someone else is getting blacklisted from whatever they're doing on that same IP. If it's not a virus or whatever, then you likely got an IP address that was assigned to someone spamming someone.

Afterwards I'd call your internet provider because although you're supposed to get a new IP address every week or so, they typically assign you the same one. Making a call could likely get you a new one issued due to your situation.