Hacked? – Weird sites pop when I log in

My production site, lifering.org, runs on a GoDaddy linux virtual server. In the last couple of weeks, when I try to log in, I may get one or several sites pop in my browser, nothing I want: sex sites, sites selling me products I didn’t ask about, and today one tried to download a video player to my desktop. I did not accept.

We must have some kind of adware on the site but I have no clue how to start looking for it. Suggestions? I cruised through the documentation but all the titles suggest they’re about “hacks” that I may want to make, not hacks someone has made on me.