Welcome at HCM.
I have read complete details of your question.
It looks from your details that you are having pain in back which increases on sitting.
Such type of pain can be most probably due to disc problems in spine or due to arthropathy or joint problem at spine.If you are over weight then this can also be one of the reason for that.Muscle weakness
aggravate this problem.
Pain due to kidney
problem radiate from back to groin.There may be associated burning during urination or blood in urine.
At present your should do following to have relief in this-
-Avoid lifting heavy weight.
-Start doing exercises as described in following web link-
-Do warm fomentation over the back with red rubber bottle which is available OTC.This will give muscle relaxation.
-Take a mild analgesic
-Have complete investigation including x ray
of affected part to rule out any problem.The MRI will be best for this.
Hope this all will help you.
With best wishes.