Diabetic Ketoacidosis (DKA): The child's symptoms of severe nausea, vomiting, abdominal pain, and increased urination a few days prior to the onset of the current symptoms, followed by a cessation of these symptoms, could indicate a diagnosis of DKA. The recent history of "cold symptoms" and fever treated with OTC medications might have precipitated DKA in a potentially undiagnosed diabetic child. The physical examination findings of dry mucous membranes, prolonged capillary refill, and mild generalized abdominal tenderness support dehydration, which is consistent with DKA.

Gastroenteritis: This condition is characterized by inflammation of the gastrointestinal tract, often due to viral or bacterial infections, leading to symptoms of nausea, vomiting, abdominal pain, and dehydration. The recent history of "cold symptoms" could be related to a viral infection that has now affected the gastrointestinal tract.
Urinary Tract Infection (UTI): Although the increased urination stopped, a UTI could still be a consideration, especially if the initial symptoms were misinterpreted or if the infection has ascended to the kidneys, causing more systemic symptoms like nausea, vomiting, and abdominal pain.
Appendicitis: While less common in this presentation, appendicitis can cause abdominal pain, nausea, and vomiting. The mild generalized abdominal tenderness could be an early sign, although the lack of localized tenderness (e.g., McBurney's point) makes this less likely.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply. It's a surgical emergency and can present with abdominal pain, vomiting, and signs of obstruction. Although less common, it's critical not to miss due to the potential for severe complications.
Pyloric Stenosis: Typically presents in infants but can occasionally be seen in older children. It involves the narrowing of the pylorus, the opening from the stomach into the small intestine, leading to projectile vomiting and dehydration.
Pancreatitis: Inflammation of the pancreas, which can be caused by various factors, including viral infections, and can present with abdominal pain, nausea, and vomiting.

Adrenal Insufficiency: A condition where the adrenal glands do not produce adequate amounts of hormones, potentially leading to nausea, vomiting, abdominal pain, and dehydration, especially in the context of a recent illness.
Hemolytic Uremic Syndrome (HUS): Typically follows a bacterial infection (like E. coli) and can cause acute kidney injury, leading to symptoms that might initially seem similar to those presented, including abdominal pain and signs of dehydration.