The lump you refer to may represent a number of possibilities, some serious, others not. It also depends on the type of umbilical hernia repair
that was done. In most cases, a piece of mesh is inserted to bridge the defect in an umbilical hernia. The lump may represent soft tissue swelling
from surgery, a hematoma (blood collection), a seroma (fluid collection), an infection, failed hernia repair with a piece of fat or intestine
that has gone through the hernia defect. The former three are usually self limiting, however the latter three are serious and may require treatment. The only way to distinguish between these possibilities is by seeing the surgeon and if needed, having tests, such as an ultrasound or ct scan
to be done.