Hi, sounds like when you hit the back of your head on the grass your brain may have gone backwards hit the back of the skull then bounced forward to the front - coup-contrecoup injury it's called. The brain swells with bruising, it can take time for the swelling to come up and go down completely in the meantime you could get different symptoms arise including a feeling of pressure in the head. I would take it easy, no strenuous activity and quit alcohol (it dehydrates the brain) until you are completely better.