Is the bulge under my belly button due to swelling, seroma, or poor liposuction? (Photo)

At the moment I don't think you have anything to worry about. There is definitely significant swelling it seven weeks. Be patient and keep in contact with your surgeon.

Without an exam, it is difficult to say for sure. Being that you noticed it immediately, it can be any of the above you mention. If the area is firm, it is most likely some scar tissue. Scar tissue can build up in areas treated with liposuction to various degrees but that usually begins after about 1 week after surgery. Lumps of scar tissue can take 4-6 months to resolve. You plastic surgeon is the best person to answer your question. Minor contour irregularities commonly occur after liposuction and can take months and months to improve.

You definitely still have some swelling left at 7 weeks, however since your contour is so nice and smooth I would guess it is some residual fat. But the best course of action is to communicate with your surgeon.Good Luck.

The best way to diagnosis for issue is an IN PERSON examination, a scan of the area. The possible causes are residual fat, scarring, seroma.. Or a combination of these. My gut is guessing poor liposuction technique. 

It is difficult to say without seeing you . The bulge can be swelling, fluid, weak muscle tone, to name a few. Best to be seen in person.