I used to experience this except it was in the morning. A feeling of complete dread would overcome me, just a full body feeling of fear...probably anxiety. It probably goes back to fight or flight type of things in our brain...I'm not totally sure. But for me similarly, once I woke up and started my day the feeling would go away and I could think normally. Now I rarely experience anxiety of any kind for some reason. It disappeared a while ago. I think therapy helped a lot for me.