Cortisol is a horomone- however if you are low in it in some adrenal disease like addisons disease you will need to take a replacedment steroid. Cortisol is the main horomone that has reduced in function with your adrenal cortex (little glands above your kidneys) aren't working properly. There are a few causes to this. However, if you do have this you will need to take replacement of this horomone in order to be healthy for the rest of your life.
Its a class of glutocortoriods. Medication for this condition is usually administered as an injection, unless otherwise. Its not to be confused with topical cortisone. They act different in time and break down in the body.
Cortisol is normally occuring in the body to regulate your sleep and waking cycles as well as body functions in temperature and immune function. If you are low in potassium you will have low cortisol. Its a very delicate balance. If this isn't working well, you dont fight infection well, feel fatigued constantly and can have a host of other system aliments. Especially you become dehydrtated easily, especially when sick because you bodies potassium/salt mixture is altered causing low blood pressure.
It is hard to diagnose as there is no straight test, however once found the treatment is very straightforward. However, you sound like you present with very known symtoms for the disease.
This can also become pronounced in women who have entered menapause or have had a very stressed life, smoke and heavy stimulant use. Whatever you do not consume coffee or chocolate, pork/ham. This makes your problem worse because it strains your adrenals that make cortisol to make you feel worse.
Rest, moderate exercise, proper eating and meditation techniques are vital to your well being with this in making you feel better.
There can be many things in the gland system going on for you despite the evident- you could also be fighting an infection and your body isn't responding properly. If anything I would make sure you follow a proper hypoglycemia diet to help out your system if you aren't eating healthy already and hopefully you will have a good endocronologist you meet with that can guide you well. If you have any questions just ask.
Goodluck!
Kristina