Try wearing looser fitting clothes. Specifically, loose fitting under shirts. This helps reduce the rubbing of the cloth against the skin as you move. Also, buy under garments without polyester in them. Most t-shirts you can buy these days have as much polyester as cotten. Look for ones higher in cotten, lowerin polyester. Your shirts tend to have the highest contact to your skin on the upper back/shoulders and the outside of your arms near your shoulders, You may have a mild allergy causing those bumbs where your skin has the most contact with polyester. Don't laugh, allergies to polyester are common.