I'm sure that this hard, movable and painless lump is fibroma, a benign tumor that rises from the fibrous tissue located in dermis (the thicker and deeper part of the two parts of the skin).
Fibromas are very common growths, quite harmless and will not turn into a cancer.
They are usually, painless, but, occasionally, can itch or hurt when touched or knocked.
Fibroma, often, feels like small rubbery button lying just under the surface of the skin, with 1 cm across.
It can appear in the armpits and groin areas as well as on the back skin, and it's development is triggered by microtraumas, scratching or insect bites.
This is just an aesthetic problem.
Best wishes!
Marija