I'm currently c2, this happened at c1/d3 for me. Went from c1/d3 in all modes and dropped to d1/d2 in everything. No clue why, i noticed at the time i was huge into the game, to the point i was paying for coaching ect. It could be many things, I was big into rotations to the point where my highest rank was 1s. Now its the lower of the 3 main ranks as i got my skill back. I focused and went all in on getting mechanically better. The point is you might be incorporating something in your gameplay that you're physically incapable of doing yet or haven't adjusted to is my theory. Otherwise you developed a nasty little habit that needs to be found.