After drinking prune juice, had a large bm after dealing with constipation for weeks, now lower back is sore and or in pain, that normal?

Constipation: Can certainly cause back and abdominal pain. This should resolve on its' own- if not, see your physician.

Could be...: It could be that after having constipation for so long your abdominal organs are still sore. Make sure you keep eating a diet high in fibers and water so if there is still some hard stools contained your body disposes of them. If you develop severe abdominal pain or get sicker make sure you get seen by your doctor.