Why is my screen being bombarded with ads?

For the past couple of weeks, my phone's screen has been invaded with ads. Turning on airplane mode gives me a temporary fix, as well as just turning of the WiFi and mobile data. I think it might be a virus due to the fact that no matter how many times I disable the 'can appear on top' option on every app, the problem persists, but if it's a virus, it is somehow able to hide from the three anti virus apps I have on my phone. The ads take over the whole screen and usually don't relate to what I've been doing on my phone, although occasionally, I get a few ads that relate to my recent online activity. The ads always lead to a secure sight, so I'm not too worried about that, but it's still annoying as all hell. If anyone knows what's going on and/or possible fixes, please let me know.

Thanks in advance.