Constipation: Given the patient's hard bowel movements for 2 days and low water intake (2-3 cups daily), constipation is the most likely cause of the upper left abdominal quadrant pain. The fact that Miralax (a laxative) was given also points towards this diagnosis.

Gastroesophageal Reflux Disease (GERD): Although the patient does not have nausea or vomiting, GERD can cause upper abdominal pain that may be referred to the left upper quadrant.
Peptic Ulcer Disease: Peptic ulcers can cause intermittent abdominal pain, and the location of the pain can sometimes be in the left upper quadrant, especially if the ulcer is located in the duodenum or the stomach.
Diverticulitis: While more common on the left lower side, diverticulitis can occasionally cause pain in the upper left quadrant if the diverticula are located in the upper parts of the colon.

Splenic Infarct or Rupture: These conditions can cause severe left upper quadrant pain and are medical emergencies. Although less likely, they must be considered, especially if the patient has a history of trauma, sickle cell disease, or other conditions that predispose to splenic issues.
Pancreatitis: Pancreatitis can cause severe upper abdominal pain that radiates to the back and can be a life-threatening condition if not promptly diagnosed and treated.
Pneumonia (Left Lower Lobe): Sometimes, pneumonia can cause referred pain to the abdomen, and it's crucial to consider this diagnosis, especially if the patient has respiratory symptoms or risk factors for pneumonia.

Gastric Volvulus: A rare condition where the stomach twists abnormally, causing severe upper abdominal pain.
Intestinal Obstruction: Although more common in the lower abdomen, an obstruction in the upper gastrointestinal tract can cause pain in the left upper quadrant.
Left Diaphragmatic Hernia: A rare condition where abdominal contents herniate through the diaphragm, causing upper abdominal pain and potentially life-threatening complications if not recognized.