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?

Yes: Plain x-ray might show obstruction. Best test is a ct but in your case might not be needed.

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.