Sure. I dont think it is any more or less likely than having not been on chemo.
Yes. Many chemotherapy agents can cause muscle and backe pain, which may persist for a while after completing therapy.
Something else? It depends, but one of the commonest causes of back pain after chemo is bone marrow stimulation if a medication like Neupogen or Neulasta (pegfilgrastim) has been used to prevent a drop in counts. Please check with your doctor if that is the cause and to make sure you do not need an xray, a bone scan or MRI to rule out something else as the cause of your back pain.