Since your investigations were normal, it implies there is no serious underlying cause of your symptoms. Normally breathing happens without our noticing it but if there is a mild blocked sensation, then it comes to our attention. It is possible, there is a mild inflammation ( as you report sticky mucous and mild nasal obstruction) which is causing you to become aware of it and therefore a subconscious effort to increase your oxygen intake. A detailed history may help identify any other cause for your symptoms.
Next Steps
Since you are anxious about it, please consult a doctor and discuss your symptoms in detail to identify any underlying cause
Health Tips
Plain steam inhalation 2-3 times a day, Sinus rinsing ( Jala-Neti) may help in the meanwhile