Why is there a hard knot near the belly button after bowel obstruction surgery?

Depends: What type of surgery did you have; open or laparoscopic? Is there a healing incision at the site? I assuming the firmness that you feel is at the site of a healing scar. It will resolve over a few months. See your surgeon for an exam to be sure that you are healing properly.

Scar tissue: Unless there are signs of infection, the most likely reason is the formation of scar tissue. Another possibility is a hernia. You should see your surgeon to evaluate.