- A ghost or spirit that is trying to get your attention.
- Psychic phenomena -- a physical manifestation arising out of the subconscious, which many researchers believe accounts for poltergeist activity.
- Those who believe in demons will blame them. (I don't.)
Which is responsible in your case? It's difficult to tell, especially since you haven't disclosed whether or not you have experienced other poltergeist or haunting activity in your home.
There is another possibility in your case, which has to do with your martial arts experience you mentioned. It's possible that as you lay asleep in bed that you experienced a strong muscle spasm (or some other explainable physical pain), and in your sleeping state you had the body memory of a forearm strike, which you knew from your martial arts days. Your unconscious mind put the pain and the memory together, and since there was no one there to actually deliver such a blow, it seemed as if a spirit was responsible.
See the links below for other stories of ghost attacks.