Hi Doctor, Ive Been Experiencing This Issue Of Back From

Hi Doctor, Ive been experiencing this issue of back from quite few years. Which started with a small area in the center of my back and then spreaded towards my shoulder little on my waist and at times in my chest near the nipple area.
Doctor suggested me propygenta cream and when applied it goes off but as soon as I stop using it it gets started once again. The itchy get so desperate that I bleed from those tiny bumps.
What is the possible reason for this situation.