Hello... I went through your symptoms. As you mentioned, the pain under the rib cage could be because of myo fascial pain which has already been taken care of by flexeril. Physiotherapy and stress relieving activities will also help in improving symptoms of myofascial syndrome. The most common cause of diarrhoea is food poisoning. If you have had food and drink from a new place, that could be the cause for diarrhoea. Food rich in iron or iron supplements could lead to blackish stools.Also sausages made from animal blood can lead to black coloured stools.so, try and recall if any of these events took place, which could have caused this condition. From your history, I understand that you have been diagnosed and are on treatment for GERD. If you do not have any events relating to food poisoning or consumption of iron rich food,then upper gastro intestinal bleeding could be suspected in your case, as gastro oesophageal reflux (GERD)causes erosion and bleeding in the oesophagus,which leads to black tarry stools.If you have had any trauma to your nose that led to nasal bleeding,swallowing that blood could lead to black stools. You must consume a lot of fluids to prevent dehydration from diarrhoea. If it is due to food poisoning, it should be controlled in 2 days.If there is very severe diarrhoea causing dehydration,then you must seek immediate medical help for intravenous infusion of fluids. Flexeril is also known to cause diarrhoea in some patients. The following investigations are required if there is continued discolouration of stools- A complete blood count, upper gastro intestinal endoscopy to rule out bleeding, stool examination for presence of blood. All of the above mentioned problems can be resolved with timely medical intervention. I hope this was informative. Take care.