Multiple myeloma is a type of cancer that affects the plasma cells within the bone marrow. It damages the bones and inhibits the body’s ability to produce healthy blood cells. In healthy people, these plasma cells (a type of white blood cell) help the body fight against infections by making antibodies that attack germs. 
The plasma cells affected by myeloma become cancerous and eventually crowd out the healthy cells in the bone marrow. This causes the cancerous cells to produce abnormal proteins instead of antibodies.
The cause of multiple myeloma is unknown. There are no known risk factors either, but researchers suspect that certain genetic abnormalities play a role in the development of this disease. 
Multiple myeloma has no cure.  Treatments often focus on alleviating the symptoms associated with the disease using medication.  Platelet transfusions, plasmapheresis, and stem cell transplants may also be employed. If the myelomas are discovered early, treatment is often delayed until the disease starts to show symptoms. 
Cancers do not often show any signs or symptoms until the disease has progressed significantly. Multiple myeloma is like this. It occurs in multiple stages, with the second and third (terminal) stages being the ones where symptoms are likely to start showing.  This article explains some of the symptoms you may notice if you have multiple myeloma.
Bone Pain or Tenderness
Bone pain is an aching or tender discomfort felt in one or more bones of the body. Unlike pain in the muscles or tendons, the pain persists whether one is moving or not, and the pain is unmistakably skeletal. Many types of health conditions can cause this problem, including multiple myeloma. This is because myeloma cells grow in the bone marrow. Myeloma cells increase the activity of osteoclasts (these break down bones) and reduce the activity of osteoblasts (these form bones), causing bones to break down faster than they are formed. 
If you have multiple myeloma, you may start to feel pain in your bones. This pain may be present while you’re resting or when you apply pressure on certain body parts (tenderness). While this can affect many bones, people often feel the pain more often in the spine and ribs.  The skull and bones of the feet are also commonly affected.