Constipation: Given the symptoms of abdominal cramping, lack of appetite, and increased fatigue, constipation is a common and likely diagnosis in children, especially after a significant medical procedure like heart surgery, which can affect bowel habits due to medication, reduced mobility, and dietary changes.

Gastroenteritis: This is a common condition in children that can cause abdominal cramping, loss of appetite, and fatigue. It's often viral but can be bacterial, and the recent history of heart surgery might increase susceptibility due to potential immune system suppression.
Medication Side Effects: The child is on Lasix (furosemide), a diuretic used after heart surgery to manage fluid balance. Side effects can include dehydration, electrolyte imbalance, and gastrointestinal upset, which could explain the symptoms.
Postoperative Adhesions or Bowel Obstruction: Although less common, it's possible that the child could be experiencing complications from the recent surgery, such as adhesions or a bowel obstruction, which would cause severe abdominal pain, vomiting, and constipation.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply. It's a medical emergency and can cause abdominal pain, vomiting, and bloody stools. Although less common, it's critical to consider due to the potential for severe consequences if missed.
Appendicitis: Inflammation of the appendix that can cause severe abdominal pain, fever, and loss of appetite. It requires prompt surgical intervention to prevent rupture and peritonitis.
Infective Endocarditis: Given the recent heart surgery, there's a risk of infective endocarditis, an infection of the heart valves. Symptoms can be non-specific, including fever, fatigue, and loss of appetite, making it a critical diagnosis not to miss.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic abdominal pain, diarrhea, weight loss, and fatigue. While less common in children, especially without a family history, they should be considered if other diagnoses are ruled out.
Celiac Disease: An autoimmune reaction to gluten that can cause abdominal pain, diarrhea, fatigue, and weight loss. It's less likely but should be considered, especially if there's a family history or if the child has been experiencing persistent symptoms.