Your condition seems like ingrown hair. It's a benign , very common condition.
There are small lumps at the back of the arm , shoulder, back and buttocks at the hair follicle due to repeated friction from the fabric.
Next Steps
-wear softer fabrics, loose fitting clothes.
-wash the area with soap and maintain hygiene to prevent follicles getting infected.
- don't squeeze the lumps , which increases chances of infection.
- if there is pain, increase in size , itching or pus formation visit nearest dermatologist
Health Tips
consult for more details.