Biologically, we are not designed to stand upright.
That's why we have so much trouble with our spines, legs and feet, because un-natural pressures are put onto bones and ligaments not deigned to take them.
That's why your back pain could be any one of a number of thjings - although sciatica is the most likely.
Your doc. may refer you to a phsyiotherapist who will sort out the root cause of the pian, and treat it, but in the mean time, try to sit up as straight as you c an, and bend your knees not your back if you need to pick anything up from floor level.
Good luck.