Thanks for the feedback and playing my game!
I can be sensitive to low frames which can evoke the same feeling in me, and I am simply updating the position every frame to exactly where my mouse is, which does cause not smooth cursor movement, but did not know it could cause the kind of nausea you mentioned.
I used a dynamic camera and put some code to make it work with controller and the code is not very nice, which I think is the weird behaviour I think a delay would be nice too. The camera with the fast movement is a bit weird! In fact I had to put a delay in the shotgun animation so the animation clearly visible, because in motion it isn't.