What can cause elevated pm cortisol levels?

Cortisol: Blood cortisol varies throughout the day. Although it tends to be higher in the am than the pm, it is pulsatile, and responds to stress. So it is not a simple matter to even define an elevated pm cortisol. Cushing's syndrome elevates both am and pm cortisol and removes the circadian pattern. You don't mention why this was measured. Suggest seeing an endocrinologist.