Hi
The advice is always to speak to the medical experts or the Myeloma helpline if you have any concerns.
Your not alone. Lack of mobility seems to be a common side effect of Myeloma either due to bone damage, problems with the spine and spinal cord, and muscle weakening as a side effect of the medication.
I had all three when first diagnosed but am now mostly sorted and enjoying an improved quality of life.
Hope this helps.
David S