Well from what you are describing there are a few conditions which can cause this sort of issue. The presence of vomiting in the beginning followed by constipation could be an indication that you could have developed acute intestinal obstruction. This is a condition in which there is a blockage in the intestines which could be excarebated by the drugs that you inject. This would a distended abdomen, tenderness (not always though) and absolute constipation. if this is the case then you need to have this evaluated ASAP by getting a abdominal xray done as well as an ultrasound. This should be done in the ER ASAP.
Another cause for these symptoms could be that you have developed acute pancreatitis as patients who inject drugs are at an increased risk of developing inflammation of the pancreas. This would cause abdominal pain, tenderness, distension as well as constipation in some cases. The diagnosis would be made with a CT scan of the abdomen as well as blood tests like lipase and amylase. The treatment is with IV fluids and antibiotics.
Lastly the upper abdominal pain as well as the constipation could be indicative of gastritis. This is a condition in which the lining of the stomach is inflammed which can also cause burning pain in the chest, metallic taste in the mouth and even blood in the vomitus or black colored stools. It is also possible that you could have developed a stomach ulcer which is now causing the symptoms. The diagnosis here is with a clinical examination and if needed an endoscopy.
Given your history of hepatitis C i would also like to exclude acute liver issues which can cause this sort of pain, now i would suggest that you use OTC prilosec for now but you need to go to urgent care or the ER to have this evaluated ASAP.
Do you have any more questions?
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