What would cause my pulse ox levels to go from consistently 99 or 100% to being 96 or 97% consistently? need for concern?

Does one smoke?: 31 y/o female asks What would cause my pulse ox levels to go from consistently 99 or 100% to being 96 or 97%. First, Is person currently acutely ill with cough/ Upper respiratory Infections/ Pneumonia or muscle strain/injury that restrict full lung expansion and deep breathing. 2nd, does one Smoke cigarettes or other items. Sedentary life/lack of exercise/ weight gain can also come into play