[GAMEPLAY..?] Champion refusing to do what i want it to do

As the title says , last 3 or 2 games i was playing Riven and sometimes (noticed only where there are enemy or hostile units that i have on my screen like jungle monsters , enemy minions etc , you get it) the champ would just REFUSE to do anything like ANYTHING.Let's say I click a minion because i can perfectly last hit it in that exact time I clicked it , nope , the champ's not moving , doing the action only after 1 or 1.5 seconds or sometimes even more and missing 1 cs.Same thing happens over and over and over , last game i lost more than 50 cs just because of that and also died multiple times because the timing on the skills / summoner spells wasn't right.. I hope you can help me and tell me what caused this/how to stop it. P.S:I'm not sure if this is a bug or i should post this in the Help section