When I join a world, my player model spazzes out and the arms and legs start flying everywhere. Furthermore, when I try to run forward it pans way far out to the point where I can't see anything and zooms back to normal when I stop running.

It's been happening every time I launch Rebirth of Night today. It worked fine until I installed optifine 1.12.2 F4, but since then it has been crazy.

I expected it to work as usual with normal controls

Info (please complete the following information):

• Modpack Version: 2.76.1
• Server or Singleplayer: Server
• Version the world was created on: 2.75.3

Additional context
It worked fine without optifine, but I need optifine to apparently run the pack and get passable FPS

It doesn't seem to be letting me attach a video

This is an issue with forge/minecraft. The solutions for it are on google.