Suspected leak: The clinical presentation of abdominal pain, generalized tenderness, distension, and fever on post-operative day 7 after an anterior resection is highly suggestive of an anastomotic leak. This complication is a well-known cause of post-operative fever and sepsis in such patients.

Subphrenic collection: A collection of fluid or abscess in the subphrenic space can occur after abdominal surgery and cause fever, abdominal pain, and tenderness. Given the recent surgery, this is a plausible cause of the patient's symptoms.
Pulmonary embolism: Although less directly related to the surgical site, pulmonary embolism is a significant risk in post-operative patients due to immobility and potential for deep vein thrombosis. It can present with fever, among other symptoms, and should be considered in the differential diagnosis.

Sepsis from other sources (e.g., urinary tract infection, pneumonia): While less directly related to the surgery, other sources of sepsis can present similarly and are critical to identify and treat promptly to prevent severe outcomes.
Intra-abdominal hemorrhage: Although less common as a cause of fever, intra-abdominal bleeding can lead to sepsis and is a life-threatening condition that requires immediate intervention.

Stump appendicitis: If the patient had an appendectomy during the anterior resection, stump appendicitis is a rare but possible cause of post-operative fever and abdominal pain.
Intra-abdominal foreign body reaction: A reaction to a foreign body left in the abdomen during surgery, such as a sponge, can cause fever, pain, and other systemic symptoms, although this is exceedingly rare with modern surgical practices.