Without seeing you to talk about your pas experience (i.e. medication you are on, or past psychological history) it is hard to tell. If nothing is wrong it's very unusual that this is happening. You might have a possible medical problem with lying on your back. You might have some type of post traumatic stress syndrome and it is linked to lying on your back. There are so many different possibilities, you should see a medical doctor or psychologist to discuss what is going on in your life in more depth.