This rather does sound like Niemann-Pick disease, but it is very rare and does not really occur in adults. It usually begins before the child is even born! Sadly, they don't usually make it into teen years. My cousin has this (8 years old) and she is dying. But they may need to do some blood tests to make sure no autoimmunity or cancers are going on. If your spleen does not retreat to normality, I think you should start to be concerned.
They've ruled out all these?
What causes it?
A number of infections and diseases can contribute to an enlarged spleen, including:
Viral infections, such as mononucleosis
Bacterial infections, such as syphilis or an infection of your heart's inner lining (endocarditis)
Parasitic infections, such as malaria
Cirrhosis and other diseases affecting the liver
Various types of hemolytic anemia — a condition characterized by premature destruction of red blood cells
Blood cancers, such as leukemia and Hodgkin's disease
Metabolic disorders, such as Gaucher's disease and Niemann-Pick disease
Pressure on the veins in the spleen or liver or a blood clot in these veins
Mononucleosis,Syphilis,Endocarditis,Malaria,Cirrhosis,Leukemia,Hodgkin's lymphoma (Hodgkin's disease)Gaucher's disease.