It could be anything! How about going to the doctor and have it checked
You should possible get an xray to make sure you have not cracked or broken anything. It is possible to run into things or get bumped and not even notice how badly you are hurt at the time.
I would certainly see a doctor and a chiropractor. I get spams similar to what you are talking about in my back and neck, the adjustments help a lot.
Also, try soaking in a hot bath with epsom salts, this helps to relax the muscles some.