It seems very severe if you cannot keep even pills down. If you don't have any other symptoms besides abdominal pain, then this could be related to a severe viral illness. Usually these are self limited, and clear on their own within 24-48 hours, however the goal is to keep yourself well hydrated in order to prevent complications of dehydration (such as renal failure, electrolyte disturbances, confusion, etc). Headache can be an early sign of dehydration. So if you cannot keep fluids / nothing down, then you probably should go to the emergency department to get an intravenous medication for the vomiting, get some IV fluids for re-hydration. They will discharge you as long as you can keep liquids down. Then at home you can use suppositories for vomiting, or pills which is obviously more comfortable. If the abdominal pain is worsening then go straight to the emergency as this could mean something more severe than just a viral illness. Keep in mind that retching and vomiting a lot can give you upper abdominal discomfort.
If this is something that has happened recently in the past multiple times, then you should seek expert care, and you would possibly need an upper endoscopy to look at the stomach, however if it's only for 15 hours, I would not, and guide any further testing based on a physical exam.
Hope that helps. Let me know if you have any further questions or concerns.

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