Does my computer have a virus?

[Lea]

Yesterday I started noticing that my internet was acting weird. Whenever I search for something or pull up facebook or Twitter there are all these ads that aren't normally there. I know sometimes there will be ads on the side but these are at the top of the page and make the content I'm actaully looking for towards the bottom. And it keeps asking me if I want to display secure content. It's never done this bfore. It also says ads not by this site so I don't know why they are showing up now.