Why does it hurt when I have to raise my arms to do something?

Shoulder strain?? The symptoms you describe could be due to a shoulder strain. See your family doctor, internist, or orthopedic specialist to confirm your particular diagnosis.
Rotator cuff??? Unsure without a history. I work out a lot and sometimes get those shoulder symptoms. The pain often lets me know that I have to alter what and how I am doing the exercises. I would first be checked by a specialist.