Why does fluid build up after hernia surgery?

Fluid: Fluid (called seroma) can build up in the empty space where the hernia had been. Also the tissues around the surgical site can become temporarily swollen and firm. This fluid formation is common, is more noticeable after repair of larger hernias, and resolves over several days to a few weeks.

It's normal: Typically, when the contents of the hernia are returned to the abdominal cavity, there is an empty space there. This space collects a variable amount of fluid in it that absorbs back in to the body over several weeks to months. This is a normal part of the healing process.