Post-infectious irritable bowel syndrome (IBS): This condition often develops after a bout of food poisoning, leading to persistent abdominal pain, changes in bowel habits (such as constipation), and cramping. The fact that symptoms started after food poisoning and have persisted, despite treatment with Miralax, supports this diagnosis.

Constipation with impaction: Despite the use of Miralax, if the constipation is severe, it could lead to fecal impaction, causing lower abdominal pain and cramping. The failure of Miralax to resolve constipation might indicate a more severe form of constipation.
Gastrointestinal infection sequelae: Some gastrointestinal infections can lead to prolonged symptoms due to inflammation or persistent infection. The ongoing pain and constipation could be a result of the body's response to the initial infection.
Inflammatory bowel disease (IBD) exacerbation or onset: Though less common, the onset of IBD (such as Crohn's disease or ulcerative colitis) could be triggered by an infection, leading to chronic abdominal pain, changes in bowel habits, and other symptoms.

Appendicitis: Although the history of food poisoning might suggest a less severe condition, appendicitis can present with lower abdominal pain and changes in bowel habits. It's crucial to consider this diagnosis due to its potential for severe complications if not promptly treated.
Intestinal obstruction: This is a serious condition that could cause severe abdominal pain, constipation (or obstipation), and vomiting. Any sign of obstruction, such as severe pain, vomiting, or absolute constipation, warrants immediate medical attention.
Diverticulitis: Inflammation of the diverticula in the colon can cause lower abdominal pain, changes in bowel habits, and potentially serious complications if not treated.

Colonic stricture: A narrowing of the colon that could be caused by chronic inflammation or scarring, potentially leading to obstructive symptoms like constipation and abdominal pain.
Neurological disorders affecting bowel habits: Certain conditions, such as Parkinson's disease or multiple sclerosis, can affect bowel function, leading to constipation and abdominal pain, though these would typically be accompanied by other neurological symptoms.
Endocrine disorders: Conditions like hypothyroidism can cause constipation and potentially abdominal pain, though other systemic symptoms would likely be present.