This sounds like allergic rhinitis triggered by something specific to your home, most likely dust or a stored mattress or bedding, rather than mold or outdoor pollen, since it only happens there and continued even after you replaced the mattress.
Next Steps
Wash bedsheets, pillow covers and curtains weekly in hot water, use allergen-proof covers, and air out the room before you arrive. An antihistamine can ease symptoms while you're home, and allergy testing can pinpoint the exact trigger if this keeps recurring every visit.
Health Tips
See a physician or allergist if the sneezing is ever paired with wheezing or breathlessness.