A lone hand in the dark

Nothing much happened... Im an atheist and i dont believe in the paranormal, there is a logical answer for everything, but here is my story.

I was sitting by the computer. This was 2 weeks after my dad had passed away. My mother was downstairs getting ready to go to bed (could hear her rocking about in the bathroom thru the music in my headphones). Some minutes pass by and i feel a hand touch my shoulder, like real physical wheigt being put on my shoulder, a whole palm. Not pushing, just resting there. My parents always did this so that i dont get scared when they announce that they are going to bed.

I was in the middle of a bossfight in WoW so i couldn't turn around at the same moment the hand was put on my shoulder. After 5-10 seconds i remove my headphones, spin my chair around and get ready to rise up to give my mother a good night hug. But the problem was that no one was in the room, and my door was still closed.. And after a few seconds i could still hear my mother downstairs. I went out in the hallway and shouted to her asking if she was in my room a few seconds ago, and got a very confused 'no?' as an answer, I didn't debate this with her further.

When i came back to my room i just sat down and stared into nothingness. I KNEW someone had put their hand on my shoulder but no one was around. I could still feel the touch. This episode has haunted me for 5 years now and i can still go back do that very moment but i can't find an answer for what happened.

Sorry if there is any gramatical errors, I'm not english and im too tired to proof-read.