I have a cough that comes when Im sleeping. What could it be?

It's not a sore throat. My throat hasn't hurt at all. I can't sleep!!!! And I don't have allergies..My boyfriend said it could be allergies, but I don't any other symptoms of allergies. When I cough it gets so bad I feel like I have to throw up then I try and nothing comes up. The phlegm has no color, everytime I've tried to get some out..What do you think?