Viral Gastroenteritis: This is often the most common cause of abdominal pain in the context of small leukocytes (leukopenia), as viral infections can suppress bone marrow production, leading to a decrease in white blood cell count. The symptoms of viral gastroenteritis, including abdominal pain, diarrhea, and vomiting, align with the presentation.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause abdominal pain and may be associated with leukopenia due to chronic inflammation and possible bone marrow suppression from chronic disease or medication side effects.
Viral Hepatitis: Hepatitis viruses can cause abdominal pain and may lead to a decrease in white blood cell count. The liver's role in immune function and the potential for hepatitis to affect bone marrow adds to the plausibility.
Intestinal Obstruction: While not directly causing leukopenia, intestinal obstruction can lead to abdominal pain and, if caused by a condition that also affects the bone marrow (like lymphoma), could be associated with low white blood cell counts.

Sepsis: Although sepsis typically presents with an elevated white blood cell count (leukocytosis), in some cases, especially in immunocompromised patients or those with certain infections (e.g., neutropenic sepsis), it can present with leukopenia. Sepsis is a life-threatening condition that requires immediate attention.
Appendicitis: While appendicitis usually causes an increase in white blood cells, early in the course of the disease or in certain populations (like the elderly or immunocompromised), it might not. Missing appendicitis can lead to severe consequences, including perforation and peritonitis.
Neutropenic Colitis (Typhlitis): This condition, often seen in neutropenic patients, can cause abdominal pain and is a medical emergency due to the risk of perforation and sepsis.

Leukemia: Certain types of leukemia can present with abdominal pain due to splenomegaly or hepatomegaly and may cause leukopenia, especially if the bone marrow is infiltrated by malignant cells.
Aplastic Anemia: A rare condition where the bone marrow fails to produce blood cells, leading to pancytopenia (including leukopenia), which could present with symptoms like abdominal pain if there are associated infections or other complications.
Felty's Syndrome: A complication of long-standing rheumatoid arthritis characterized by rheumatoid arthritis, splenomegaly, and neutropenia, which could potentially cause abdominal pain due to splenomegaly or associated vasculitis.