Fortnite MOUSE slows down at once?

I reinstalled Fortnite today and when I wanted to play normally I noticed that my mouse suddenly became slower for a short time, then again faster then slow again etc. I have tried sooo much but nothing helps, it is NOT due to my FPS or anything else as I do Constantly 144 FPS (because of 144hz monitor) 144hz can't lie but have already tried with 60hz, also occurs only weakened.

Maybe you have your mouse set up with your mouse so a button with which you can change the sensi