According to the data you provided, you are experiencing splenomegaly. Splenomegaly might be caused by several reasons: increased function, abnormal blood flow and infiltration. The low grade fever, muscle spasms, and insomnia might be related to infective disease. Benign or malignant infiltration is less probable since it would have probably aggravated by now.
The negative tests by the specialist for infective diseases might have simply not included the real cause. Additional tests might reveal the cause.