Oxygen Level

My oxygen level is staying between 87 and 90 after getting Defibrillator.  It was around 97 before the operation.  What could be the possible reasons and corrective actions?  I am 50 years old.
The lungs are working normal according to the pulmonologist.  And I have HCM.  I feel normal with the current oxygen levels and don't feel extra fatigue.
I will appreciate any suggestions.

[Cleveland Clinic]

There are many potential causes of hypoxia including heart failure, pneumonia and pulmonary emboli, and even intra cardiac shunts. It is important that you seek emergent medical help if you feel short of breath and have a low oxygen saturation, as it may be an important clue to a serious medical problem. The placement of a defibrillator unless it caused a pneumothorax or had an unusual lead placement should not cause a hypoxia ( low oxygen saturation).