What causes of light back of the head pressure when doing laying down exercises? putting hands on back of head for support, relieves some of the pressure but slightly still there. i do abs after 2 mile walk. i feel good, no pressure when done. why?

It is likely to be due to muscular contractions associated with exercise. During exercise there are prime mover muscles that cause the movement, however, a lot other muscles are needed to stabilize the body and those muscles undergo isometric contraction. What you described is not a cause for concern. Keep up the exercise routine. Wish you good health!