Well, do you know of any known allergies you have? Or do you know of anything that seems to irritate you? It could be an allergy, but if you don't know of anything, then it could easily be something else as well. It could be debris in the air, something irritating in the fabric of your bed, or anything in your room really. That is why it will be nearly impossible to guess what is causing it since you have no leeway. IF it is even an allergy.
Since you have no ideas what it is, you should probably seek professional help. If its a big concern for you, be sure to take it up with your doctor. They can help you figure out what it is as well as give you a way to solve the problem.
Best of luck and hope you find some relief soon!