Thank you very much for the useful information Brian.
What you are experiencing is most likely acute gastroenteritis, a condition in which there is inflammation of the stomach and intestines leading to cramping abdominal pain, diarrhoea, dehydration and fatigue.
In majority of the cases, this is viral in origin and settles down spontaneously with symptomatic management (such as adequate oral intake of fluids, diet modification and over the counter remedies like Pepto Bismol). However, in some cases the infection is bacterial or parasitic in origin (such as amoebiasis, giardiasis), or arise from other etiologies like diverticulitis, malabsorption syndrome, or bacterial overgrowth in small intestine.
As per standard guidelines, you should go to ER right now for urgent medical attention including a comprehensive clinical examination, tests and IV fluid replacement. Any diarrhoea that persists beyond a week, or if becomes associates with blood in stools, weakness, lethargy or exhaustion is suggestive of serious fluid loss from the body and should be dealt promptly.
You may require a round of antibiotics as well to cure this (as viral diarrhoea is extremely unlikely to continue for more than a week), because this appears more like a bacterial or parasitic infection.
Pepto Bismol (which can turn your stool dark brown to dark black)
All will help.
But at this point, you should head towards ER (or have some one drive you there) and discuss all the details (with the doctor on call) i have mentioned to get a management plan chalked out that suits best to your needs.
I would have been worried too if I were at your place, but let me reassure you that this is a treatable condition and carries a good prognosis with appropriate management.
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