What causes what feels like pressure inside the back head when doing laying down exercises? like a headache. supporting head with hands helps relieve some of the pressure. i do abs after my walks. pressure goes away soon after i'm done. bp?

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!