What Causes Acne On Back Changing Into Rashes After Using Retin-A Micro?

Hi, I have back acne and I decided to use Retin- A micro, which I use on my face, on the back. At first everything was fine but then I got a rush. When red rush calmed down it became dark, so now instead of a few acne scars I have my back covered with huge dark spots, almost looking like burn scars.
What should I do? is it normal or I have damaged my skin?