It was a very good move on your part by calling in the fire department to check for carbon monoxide in your home. (You can also purchase a carbon monoxide detector for your home.) You can find carbon monoxide detectors for your home in a store like Wal Mart or some type of home improvement store.
Because you mentioned that the fire department have cleared the possibility of any harmful gases coming from your home and no one else is smelling it, perhaps you have an allergy or sinus infection that is being triggered by something in your home.
There is a condition called Phantosmia. This is a condition where people smell odors that are not really present. It can be caused by a few things. Damage to the olfactory (the "smelling" center of your brain) nerves in the brain can cause this. The damage is usually caused by some type of injury or viral infection, or sometimes an exposure to some type of toxin.
I suggest you see your primary physician to rule out some type of sinus infection, and if it's ruled out as the cause, perhaps he/she can refer you to an allergist. If allergies are ruled out, then possibly neurologist could be of help.
For your own peace of mind, this problem is usually caused by a sinus infection or allergy to something (perhaps to something in your home) that causes you to smell this odor, in that case is easily treatable with antibiotics or allergy medications.