Please help ! an unknown random popup in my site

a unknown popup appears every once in a while in my site…i have never been using popups ever..and i use wordpress…

the popup appears live every 3 to 7 days once..and sometimes once a every 20
page loads..(my site has lot of impressions so 20 is a high possibility)

and the popup link is
https://ads.linkbucks.com/redrockspop-sumo.html

further i use statcounter (https://www.statcounter.com/) and
whos.amung.us (https://whos.amung.us) as visitor tracing systems
but i never heard that they cause popups so i guess nothing wrong with that,

i tried to find which is the cause of this but i failed..

I appriciate if you guys can help me with this
and my site url is : https://channelslk.com

Thanks in Advance