You may have a sinus infection. My suggestion would be to consult with an otolaryngologist, often referred to as an ENT (ear, nose & throat specialist)
Do you have any other symptoms besides the unusual smell and slight headache?
Sinus infections are rountinely treated with antibiotics and decongestants.
Another consideration is a nasal polyps which can also cause a disturbance of smell. A nasal polyp is a benign tumor that grows from the lining of the nose or sinuses. They can make your nose feel stuffy, and can affect your sense of smell. Not all growths in the nose are polyps.
Click on the underlined links to learn more. Let me know what questions you may have.