Internal Hernia: Given the history of prior surgery and the presence of thickened mesentery with minimal reactive fluid in the left lower abdomen on ultrasound, an internal hernia is a plausible diagnosis. The lack of tenderness on examination does not rule out this condition, as internal hernias can be intermittent and not always cause significant pain or tenderness.

Adhesions: Previous surgery increases the risk of adhesions, which can cause bowel obstruction or intermittent pain. The thickened mesentery could be a result of chronic inflammation from adhesions.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease can cause thickening of the mesentery and abdominal pain. Although the patient's age and lack of other symptoms like diarrhea or weight loss make this less likely, it remains a consideration.
Mesenteric Panniculitis: This condition involves inflammation of the mesenteric fat and can cause abdominal pain and thickening of the mesentery, as seen on ultrasound.

Bowel Obstruction: Although the patient is not tender, a bowel obstruction, especially if partial or intermittent, could present with minimal symptoms initially. This condition requires prompt diagnosis and treatment to prevent complications like ischemia.
Appendicitis: Even though the pain is in the left lower abdomen, which is atypical for appendicitis, the condition can occasionally present with pain in unusual locations, especially if the appendix is retrocecal or has a different anatomical position.
Diverticulitis: Given the location of the pain in the left lower abdomen, diverticulitis is a consideration, especially if the patient has a history of diverticula. However, the lack of significant tenderness and the ultrasound findings might make this less likely.

Mesenteric Cyst: These are rare benign cysts that can cause abdominal pain and could potentially be located in the mesentery, although they are not commonly associated with thickened mesentery.
Lymphoma: Abdominal lymphoma can cause a variety of symptoms including abdominal pain and could potentially involve the mesentery, leading to thickening. However, this would be an uncommon presentation without other systemic symptoms.
Sclerosing Mesenteritis: A rare condition characterized by inflammation and scarring of the mesentery, which could cause abdominal pain and thickening of the mesentery.