game randomly tabbing out, then fps drops?

lately, my game will sometimes randomly tab out. even tho i'm not hitting "Alt+tab" or anything.. then when i come back into game, for the rest of the game, anytime i go to join team fights or just go to fight enemy champions in general i experience hard FPS drops, and lag.. it doesn't happen when taking objectives or even if there are alot of particles on the screen or really anything that would drop FPS.. only engaging other enemies.. is this a bug in the game, or is someone DDosing me?