Katarina cooldown bug

At some point while I was playing kat neither my Q ability nor my R worked. they both showed as off cooldown and even after I waited far past the cooldown for the Q it still refused to cast thee ability. It was only reset minutes later after I died. I notice that before It happened I had cast Q on a minion that had just died and the Q didn't go off and the Cooldown reset. I believe this to be the reason that the bug occured. I have been maining kat since her rework in 2012 never experience anything like this before. Becuase of this I think that this bug might have come from the recent change to her passive and thus her cooldowns (went from any kill or assist which is ten seconds to dealing damage in the last 3 seconds of an enemy's life). Thank you