Can an incisional hernia occur above belly button after one week of gallbladder surgery?

Possible: It may be just a collection of blood and fluid , wait couple of wks to till the wound heals and go for post OP visit to your doctor .

Yes: A hernia is simply a defect in the fascia. That defect can develop through a weakening, over time, or it can be acute. It depends on the size. I would call the operating surgeon if you do not have a post op check within the next week. Since he/she knows their technique, they would be in a better position to say yes or no.