Its worth getting it investigated as its not normal , i am not sugesting for one second you ferret has LYMPHOSARCOMA but its the only thing i can think of , off the top of my head, hope the info helps
copied this , that may be off help
LYMPHOSARCOMA
This is a cancer of the lymphatic system, which is a part of the body's immune system. It is usually found in older ferrets, but is being seen in younger ferrets now, too. Due to the younger ferret's body being geared for growth, juvenile lymphosarcoma is often fatal because the disease spreads quickly in the freshly growing cells. I