Pain in my upper arm im a hair stylist and use my arms about 12 hiurs a day what ould the cause be?

Impingement: You may be suffering from an impingement syndrome in your shoulder. This can cause lateral upper arm pain. The rotator cuff tendon can be pinched under the upper shoulder bone (called the acromion) with raising the arm and holding the shoulder in that position repetitively. The rotator cuff tendon becomes inflamed or even torn. Treatment initially involves rest, icing, and anti inflammatories.