Well...it's not all in your head! You've discovered what your bow does when you heel it.
Basically what happens is you put too much pressure low on the grip which causes the lower limb to bend more than usual. When you release, the lower limb recovers faster causing the arrow to dive down into the shelf. It will act as if your nock point is too low. High hits are common and you may see some porpoising of the arrow. Assuming you are not overbowed, and know how to grip the bow, the usual cause of heeling is fatigue.