What Causes Clear Liquid Discharge From A Skin Graft On Lower Leg?

My 11 year old had a severe burn on the inside of his lower leg along his calf where he received a graft 2 1/2 years ago,this past summer when he was at camp he was doing alot of running and jumping and his leg started to leak small amounts of clear fluid from one spot in the middle of his graft, when he came home , i applied a pressure bandage to his leg and had him elevate his leg, the next day it stopped and have had no problems until this week when he came home from school where he had been doing jumping and tumbling in gym class, he has no pain ,redness or swelling??