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I am Dr.R.K and I am pleased to assist you.
Episodic abdominal pain that wakes you up from sleep with some relief with pepto bismol could be due to peptic ulcer.
The black stool could indicate bleeding internally probably from the ulcer.
It is prudent to go to ER.
An upper GI endoscopy may have to be done to confirm the diagnosis.
Other tests that may have to be done include a complete blood count to see whether there is significant bleed, an ultrasound scan and serum amylase and lipase to look for other causes of abdominal pain like cholecystitis and pancreatitis.
Get medical attention at the earliest.
I hope that answers your question.
Let me know if I can assist you further.