Thanks for the query,
You might be suffering from acne
Though it is seen mainly on face but can even affect back.
It happens due to blockade of glands.
You need treatment with retinoids
Please meet a dermatologist
for exact diagnosis and discuss regarding start of retinoids.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.