Yes, fasting is a stress to the body and cortisol acts as general activator as well as trying to move glucose out of storage and into the blood. So, if too much cortisol is your problem, then fasting may not be right for you.
Does Fasting cause (or cure) Hunger?
Dr. Jason Fung

Dr. Jason Fung That’s why people should train during fasting periods. They will keep the cortisol level under control.