Can constipation cause pain in my right hip and lower back down to my knee or can it be sciatica? how is sciatica diagnosed?

Sounds like sciatica: The constipation would have to be long standing but if you have been straining to have a bm you could increase pressure in your abdomen that can cause a disc herniations. These are most common source if sciatica. These symptoms usually get better with time.

Well...: It depends. If you are straining a lot to defecate, you could cause disc to herniate. Also sitting a long time on the toilet can lead to increased forces on the spine resulting in possible disc bulging/herniation as well.