Invisible Force Touching My Shoulder

When I was around five, I was walking through my dad's room in the morning on a weekend. My dad was cooking breakfast, and my brother was still asleep. No one else was in the house. I remember walking towards the door when I felt something touch my shoulder. It was a pretty firm touch, as it was enough to make me believe someone was behind my. I turned to face whoever it could be, but no one was there. I screamed and ran to my dad. I have no explanation for this event other than the overactive mind of a five year old. Make of it what you will.