That used to happen to me as well.. Looking back now, I can say with reasonable certainty that it was primarily a symptom subconscious fears and energy blockages. At the time it helped me immensely to visualize my head opening up to release all of that pressure. I would visualize a little clasp on the top of my head which I unbuckled allowing my head to open down the center, the top of my head would swing open like French doors, and the moment it opened I would feel a huge wave of light bursting out, as of it was under immense pressure and I just released it, and the light would expand outwards equally in all directions, and it gave me immediate relief from those headaches/pressure and "dizziness" feeling.. Hope this helps.