Older sensors had a “max tracking speed” much lower than those in the last few years. If you swiped too fast (which is easy to do in a FPS if you’re playing at a low enough sens) it would spin out and look straight up or down like that.

With almost any sensor in any mouse using a DPI that high will introduce smoothing and other issues. I’d recommend dropping to 1600 or lower and simply raising the in-game sensitivity.