"League Client"

so i was playing got a penta killl yayy :D all of a suddden i see a league client something on the top which showed a exit button and i couldnt move the screen down with my mouse or anything everything got weird and i couldnt go scroll down some vi was talking about she hacks like tht but i dont believe her but the bug is tht it showed a pop up of a thing saying league client something and didnt want to come off.....