Help with Tumblr pop up?

[Shiloh]

Hey so I've had a tumblr for a couple years now, but recently I've got a virus on my blog I think. Whenever I go to my blog, whether im logged in or not, an ad with pop up in another window specifically an Adfly pop up. Why? How do I fix it? Its only when I type in my blog url that I get that pop up. My laptop doesnt have a virus (I have Norton Anti-Virus and it says my laptop's clean). Do you think I should change my blog theme? Maybe a virus has changed my html code to allow a pop up every time? Idk. It's only when I first type in the url. If I close it out and visit my blog later there is no pop up. And this isnt the first time it's happened - it's happened over 6 times. Argh its driving me insane.

Thanks for your help