Why does it keep turning?

i have a fancy dining room in my house that we never use like never. it just sits there and looks good. the ceiling and walls are white. the rest of the house is darker shades of grey. well the ceiling in the room keeps turning yellow!!!!!!!!! i had my hubby paint it and a few months later the white went to yellow like someone snuck in with a yellow paint brush and painted it in the middle of the night!!! i dont smoke in that room very much. i smoke in my office down the hall and in the living room and the kitchen and my bedroom but not in that room only when im walking thru it. what do you think the cause is and how can i prevent it????????????? i want white!!!