What Causes Pressure In Head After An Injury?

I wacked my head on a horizontal concrete support while ducking under it and lifted my head too early. When I hit it, it sounded almost as if the support was hollow. I had a little pain but now I feel some pressure all around my head but mainly in the middle back of the skull where I hit it. I never passed out or anything but I feel a little off. Should I be concerned? Male 24yr Thank you