If it's chronic, the cause may be a pet. Seasonal allergy recurs each year, but some years will be worse than others. Indoor molds persist year round. Outdoor molds are increased in summer and fall when there's lots of mold under damp leaves and on dead vegetation. Sometimes you can escape the cause by air conditioning, but always close the outdoor vent.
Antihistamines are the mainstay of treatment.
They can be bought without prescription. Just use the antihistamine alone. Some may need a prescription for inhaled nasal steroids. If symptoms are severe year after year, and the season is long, allergen injection therapy is worth consideration. You have to go to your doctor regularly, but this is the only treatment that changes the allergy.