You have a redirect virus. It's likely associated with an app that's installed, or something in the browser's cache. Wipe the cache (not just cookies) and look through your apps for anything that isn't 100% reputable. In some rare cases, viruses can get embedded into Android, in which case you'll need to factory reset. That's not too likely though.
(I've had one Android tablet that a customer brought me that was rendered worthless because the virus even infected the firmware, so factory resets were pre-infected, and this particular tablet had pretty much no modding community to provide a clean image.)