Hi,
Dayquill has atleast three ingredients and all of them may be responsible for what you describe here. On the other hand, did you have any preexisting diseases such as sinusitis or allergies of any sort?
Some OTC anti-allergics might help you, but the best thing would be to visit your doctor and find out what s the diagnosis.
Hope that helped.
Best regards.
is benedryl a good idea? what could cause this and what should be done?