An easy way to think of 1 G in a sudden stop would be the car suddenly being tipped upright so the front bumper is on the ground and the back is straight in the air. The force felt against the seat belts would be one G. Your dog would fly forward. The G limiting factor is your tire traction. Common tires on dry pavement cannot provide more than .9 G of stopping force, usually less.

65 mph is 95 f/sec. The normal emergency stopping time at 65 is 3.5 seconds. 95 divided by 3.5 is 27 ft/sec2. One G is 32 ft/sec2. 27/32 equals .84 G.

The whiplash force could magnify this a bit but not more than a factor of 2.

It takes 20 G's for even a super sensitive brain with a recent concussion to be reconcussed. 50 to 60 G's is usually required.

My bet is you experience a traumatic event that disrupted your sleep and may have left you with a neck strain.