FPS Problems?

Well, since yesterday I have a problem with fps. When I start a match, i have about 100fps, which is what i normally have. However when I will use an ability against another champion, or another champion will attack me, my fps drops to 20-30 and it keeps like that for the rest of the game. I am really not an expert, when it comes to IT things, so I only tried those basic things such as restarting computer or changing the quality to low/medium = As you probably know, it didn't help at all. Please guys, tell me what should I do. I'm completely {{summoner:3}}
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