Sometimes I Have Horrible Sinus Pain, When I'm Breathing In

Sometimes I have horrible sinus pain, when I'm breathing in through my nose. Breathing out is fine. My sinuses are not blocked or swollen at all. I'm having no difficulty breathing, beside the pain it causes. The only thing that helps is breathing through my mouth. Sometime the pain gets worse, then results in a headache or migraine. What might be causing this pain? thank you!