Sorry for posting here first, this was just the first website where it cropped up, but I do realize now that it's something on my computer only as all the websites I'm going to have it.
Actually it's a great place to ask a question! Otherwise you might not know what's really going on.
So...you've gotten several good suggestions; Malwarebytes is an excellent tool, I highly recommend. I also second the suggestion for NoScript if you are using Firefox, or something similar if you have other browsers.
If you are not sure about looking more deeply into your computer, I also recommend a couple of sites that have great volunteer helpers (but well-trained ones) who can look at some stuff for you and tell you if there is something more serious going on. And it's FREE. I got one of the worst "bad guys" out there in malware-land that turned my computer into a spambot. It was AWFUL. They helped me clean up my system.
I have used both Bleeping Computer and PC Pitstop, I think PC Pitstop was a little faster response time. Malwarebytes has a help forum as well, and here are others, but these guys are good.
You have to sign up for a free account, (just like here) but you will NOT be spammed or harassed.
Hopefully this is not "overkill", LOL.