Is this a hematoma or cyst after my tummy tuck?

The fluid collection is probably a seroma, which can happen after a tummy tuck. It is just a fluid collection between the skin and muscle layer. This is generally treated by aspirating the fluid, which may need to be done several times until the seroma resolves. If left untreated, the seroma can become organized, meaning the walls of the cyst thicken and treatment becomes more difficult. Other than that, there is no health risk or danger in having this problem, but I would recommend seeing your surgeon as soon as possible for treatment.