What happens when you play too much of a champion

So in the past few weeks, I spammed Zed and Viktor really hard, carried a lot of games, had high winrates and really good kdas, and now, most games is the opposite, i make a lot of mistakes with them and just get really bad kdas. Got level 7 with them but now I just suck, I played champs i haven't played for long and somehow did better. Why is this? I really love playing them but i just can't play as well as i used to a few weeks/days ago..
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