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It is most probable that what you are describing as a lump at the site of incision is a hematoma
. A hematoma is simple a collection of blood and it is a common complication following surgery.
If the hematoma is large, it would be necessary to open the incision and remove it without which there will be poor healing and infection. However, if it is small, it could be reabsorbed. The fact that there is pain and it appears large definitely requires that you meet your doctor for appropriate measures to be taken.
Hope this helps you.