Hi Mart. What you describe has happened to me at the beginning of my development. I was scared and could not quite let go. I have since learned the absolute importance of grounding yourself before meditating. Believe me, it makes all the difference, and stops you feeling out of control. I was told to imagine deep roots being sent down from my body into the centre of the earth. I then visualise them tying themselves down, coming back up , and filling my body with the goodness of mother earth.
I think that the pressure in your head is spirit trying to communicate with you. you seem highly gifted with clairvoyance. You just need to trust more.