Rituximab-induced cytokine release syndrome or infusion reaction: Although the last dose was 10 days ago, some reactions can be delayed. The elevated CRP and WBC count, along with generalized body and severe back pain, could be indicative of an immune-mediated reaction to rituximab.

Infection: Given the elevated CRP and WBC, an underlying infection should be considered, especially in an immunocompromised patient on rituximab. Common infections could include urinary tract infections, pneumonia, or skin infections.
Multiple Sclerosis (MS) relapse: Although the patient is on rituximab for MS, a relapse could still occur, especially if the drug's efficacy wanes over time or if the patient has developed antibodies against rituximab. The severe back pain and elevated inflammatory markers could be consistent with an MS relapse.
Autoimmune hemolytic anemia or other autoimmune complications: Rituximab can sometimes trigger autoimmune reactions, including hemolytic anemia. The patient's symptoms and lab results could be consistent with such a condition, although more specific tests would be needed for diagnosis.

Spinal epidural abscess or other spinal infections: These conditions are medical emergencies that could present with severe back pain and elevated inflammatory markers. Although no focal neurological deficits are mentioned, early stages of such infections might not always present with clear neurological signs.
Neoplasm: Either a primary spinal tumor or metastatic disease could cause severe back pain and elevated inflammatory markers. In a patient on immunosuppressive therapy, the risk of certain malignancies might be increased.
Osteoporotic fracture: Long-term use of immunosuppressive drugs like rituximab, along with the underlying disease process of MS, could increase the risk of osteoporosis. A vertebral fracture could cause severe back pain.

Progressive multifocal leukoencephalopathy (PML): A rare but serious condition associated with immunosuppression, including the use of rituximab. It typically presents with neurological deficits, but early stages might be nonspecific.
Hematologic malignancies: Although rituximab is used to treat certain lymphomas, there is a theoretical risk of developing other hematologic malignancies as a result of long-term immunosuppression. The elevated WBC could be a nonspecific indicator of such a condition.