Not op but cortisol is elevated in "just stress" as well as critical illness. An elevated cortisol is not a diagnostic test that can deferentiate between a serious medical problem and stress.
Also, cortisol isn't always elevated when stress is the problem. So a negative test result doesn't really tell you anything, and a positive test result tells you something, but you don't know what that something is without a lot more testing, which may or may not tell you anything.