Can constipation cause back pain? I had a chest X-ray and lumbar x ray. Would they have showed intestinal blockage? Is lumbar X-ray best?

Maybe. If sever than it can. Especially if straining to have bowel movements which increases abdominal pressure as well as pressurizing the discs. X-rays may indirectly show findings which may be suggestive but you would also need further testing and a detailed history and physical assessment to make the diagnosis.
Yes. Plain x-ray might show obstruction. Best test is a ct but in your case might not be needed.