Allergies. A change of season brings with it new allergens both indoors and out. You might want to try getting an air filter of some kind...maybe the summer heat made your house more humid or dusty or something and its effecting you. If after an air filter you're still having problems, try getting an appointment with an allergist, maybe they can help you figure it out. It could also be a number of other things like exhaustion, sleep deprivation, dehydration...need more details I guess.
I had the same thing and I realized I had been staying up till 5 am everynight and not letting my body rest...that can be the case most the time...also if u sleep laying on ur stomach its scientificly proven that you'll be more sluggish and untested...hope this helps!!