Why does the browser redirect when I am not on the internet?

sometimes i am watching some photos or doing other stuff and it goes to the browser and continues to download some files that i can not open. i dont give the permission to the phone but the download continues without my permission. i dont know how to stop it. i have the phone for two months now and i have had to reset it twice because it continues to lag and becomes very slow after two weeks of downloading these files. can you help me? i really love my phone but this thing is becoming unbearable!