Thanks for the query to H.C.M. Forum. Splenomegaly
may be due to many reasons .few of them are ,
1 reticuloendothelial system hyperplasia
as anemia, thalassemia
2 Immune hyperplasia,
viral bacterial etc
3 bone marrow
4 due to cirrhosis splenic vein
Pain may result from acute swelling of the spleen with stretching of the capsule ,infarction or inflammation of the capsule.
As you stated in your query that pain is in the back and below rib ( under also) , this pain is due to involvement of capsule of spleen.
It is very difficult to diagnose the disease on the basis of mere enlargement of spleen.
If possible upload complete blood report and various other reports as C T SCAN, ultrasound , if any.
Consult a physician and get his opinion.
Fatigue is just due to enlargement of spleen.
Hope I have answered your question.
If further any question I will help.