High Ping only in-game

My ping is usually fine, anywhere from 50-82 ms, which is perfectly playable, but suddenly, around three days ago, it began spiking dramatically to a consistent 120-250, which is basically unplayable due to the 2-3 seconds of input lag. I used the Hextech repair tool, but the ping test gave me a stable 20 ms with zero packet loss. I've tested it with a few online live ping trackers as well; they claim that it's in stable ranges until I jump in-game and suddenly it's up to the 300s. I play on NA and I know we have generally higher ping, but this literally just began a few days ago with no change in my ISP nor anyone doing a high-bandwidth process somewhere else in the house. Does anyone else have this problem?