SO, NOW FOR THE INTELLIGENT ANSWER.... Mucus
over-production is caused by an irritation such as allergy
, virus, or bacteria. It's your body's attempt to flush out the offender. In your case, the mucus is clear, seems to come and go, and doesn't come with other symptoms suggestive of infection (fever, malaise, pain, etc).
Most likely, you have simple allergies to airborne allergens.