Nasal steroid sprays like the one you were given are generally safe for longer-term daily use under a doctor's guidance, so symptoms returning 15 days after stopping doesn't mean anything went wrong. It usually means the allergy trigger is still around, not that the spray caused any harm.
Next Steps
Please see your ENT or physician again. Many people with ongoing allergic rhinitis need a longer maintenance course rather than one month on and off, and it helps to identify what you're reacting to, such as dust, pollen or pets.
Health Tips
Stop the spray and get checked if you notice nosebleeds, severe nasal dryness or crusting, since that can mean the technique needs adjusting.