Irritable Bowel Syndrome (IBS): Given the child's history of sensory processing disorder, which can sometimes be associated with gastrointestinal symptoms due to heightened sensory awareness, IBS is a plausible diagnosis. The symptoms of abdominal cramping after eating and frequent, prolonged bathroom use are consistent with IBS. The recent attitude change could be related to the discomfort or stress of dealing with these symptoms.

Functional Constipation: This condition is common in children and can cause abdominal pain and prolonged bathroom use due to difficulty passing stools. The absence of other alarming symptoms makes this a likely consideration.
Food Intolerance or Sensitivity: Some children may experience abdominal cramping and changes in bowel habits due to intolerance or sensitivity to certain foods. Given the onset of symptoms after eating, this is a reasonable consideration.
Anxiety or Stress-related Gastrointestinal Symptoms: The child's recent attitude change and history of sensory processing disorder suggest that anxiety or stress could be contributing to gastrointestinal symptoms.

Inflammatory Bowel Disease (IBD): Although less likely, IBD (including Crohn's disease and ulcerative colitis) must be considered due to the potential severity of the disease. The presence of a long-standing lymph node is noted but has been deemed not concerning by the pediatrician; however, any signs of systemic illness or failure to thrive would necessitate further investigation.
Celiac Disease: An autoimmune reaction to gluten, which can cause abdominal pain, changes in bowel habits, and other systemic symptoms. It's crucial to rule out celiac disease due to its potential long-term consequences if left untreated.
Lymphoma: Although the lymph node has been present for years and was not concerning to the pediatrician, any lymphadenopathy warrants consideration of lymphoma, especially with a recent change in symptoms or behavior.

Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, leading to abdominal pain, nausea, and changes in bowel habits.
Gastrointestinal Motility Disorders: Conditions such as gastroparesis or intestinal pseudo-obstruction can cause severe gastrointestinal symptoms but are less common in children without other underlying conditions.
Mitochondrial Disorders: Some mitochondrial disorders can present with gastrointestinal symptoms among a wide range of systemic manifestations. These are rare and would typically be associated with other significant symptoms or developmental delays.