Skillshots Shooting Opposite Direction

I have no idea what's causing this, whether it's some weird bug or some kind of mouse problem (unlikely), but there have been a fair few instances over the past couple of weeks where I will have my mouse pointed in a direction, use my ability, and the ability will go in the exact opposite direction.

Now, I normally play with quick-cast off, but when Aurelion Sol came out, I got used to using quick-cast for his q and e, since it's just much easier to maneuver with. Eventually, there came a game where I used my q, charging towards the enemy midlaner, only to get rekt when my q dies behind me and my line of travel. Then it kept happening, not very often, but enough to make me realize it was not a complete fluke. When this happens, I sometimes wonder if I somehow misplaced my mouse cursor, but I know that that is not the actual issue. When ganking bot one time as Sol, I e'd in a direction and then q'd immediately after, without moving my mouse. My q shot off straight behind me and died. (I was playing with a friend for this match, and he actually has a recording that I could probably provide, if that somehow would help.)

I thought this might be tied to quick-casting for a while, and then my non-quick-cast ult screwed me a few times by bugging out backwards. I thought it might be Aurelion Sol specific for whatever reason, and then I ult'd backwards like this as Ez in an aram. Needless to say, it's very frustrating, messing up my gameplay and making me look pretty dumb. I'm tired of it.

So here I am, feeling foolish, reporting some bug that I've never heard about and that feels like an excuse for missing skill shots. Save me rito! Either a fix or an explanation I can do something with would be awesome.

Thanks for reading :3