When doing walking felt a space at the top of the head. After done some meditation the space began to expand .What is this?And there is a kind of a heart beat like inside the space.
It's just a sign that your mind is starting to enter access concentration. It's really quite common. People also report sensations like their head swelling, body twisting, face disappearing, hands inflating, etc., etc., etc. How these feelings manifest is as unique as the person experiencing them. They're kind of like your own personal brand of samadhi entry. The sensations themselves are also not particularly important; you don't need to pay them any mind. Eventually, as your concentration deepens and you move closer to jhana, your meditation will start to take on a character more reminiscent of those described in the sutras.