On post-op day 1, tachycardia is usually from pain, bleeding or dehydration. An anastomotic leak is the least likely cause this early:

• It presents with fever, abdominal pain, prolonged ileus, leukocytosis and peritonitis, which develop over days.
• The high-risk leak sites are low rectal and esophageal anastomoses, not an ileocolic anastomosis after right hemicolectomy.

Bleeding from the anastomosis is a recognized early complication.