TC, cortisol is basically our body's antihistamine. The lower your cortisol levels are, the higher your histamine levels will be. These levels also fluctuate as we know. It could be anything setting off your sinuses. Possibly a food, but also something airborne could be the culprit. Mucous is the body's product of irritation, and histamine is a number one irritant.
I take quercetin and it has helped me quite a bit with excess mucous. Also Sea Salt , and vitamin c is a powerful antihistamine if you tolerate it