What Causes Transverse Myelitis Along With Multiple Sclerosis?

My cousin (29 y/o female and mother of 2) has been diagnosed with transverse myelitis and MS. She presented with numbness and paresthesia in her legs and feet, and in under a week it has progressed to her chest and neck. She can barely walk with the assistance of a PT and walker, and can barely text without becoming extremely fatigued. She was originally on morphine which didnt help, switched to toradol which also didnt help, and is now on oxycodone. They did an LP for Guillain-Barre which came back negative. She is now complaining of pain in the left-center of her chest. Can MS really progress this quickly??? Are there other possible diagnoses that could explain this rapid degeneration?