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Black stool can occur when people take bismuth products like Peptobismol, iron supplements and charcoal products. This is entirely harmless.
However it can also be a black or tarry stool when there is bleeding in the intestine or stomach such as with an ulcer and so new-onset black stool (especially if loose) is a reason to see a doctor as soon as possible. Any stomach pain then go straight to ER.
Although the black stool is most likely due to the peptobismol, I would recommend that you see a doctor to investigate the burning sensation in the chest. It may be that you would benefit from a medication to reduce stomach acid such as omeprazole however you may need an upper GI endoscopy at some point to check for ulcers or gastric irritation.
Adrenal fatigue is not a diagnosis that we commonly use in medicine. If your adrenal glands were not functioning then you would have a wide number of symptoms and blood tests would be able to clarify this.
Anxiety and panic attacks are very common and usually represent a mental health issue rather than a physical abnormality. Your doctor can request test for catecholamines if you wish to investigate this as a possible cause.