I Am A 50 Year Old Female. I Have Sore

I am a 50 year old female. I have sore muscles and no strength in my fore arm between my elbow and wrist. I understand this is probably due to my occupation and repetition motion of my hand. I have absolutely no strength in my arm. Example...I am unable to lift a gallon of milk with my right arm without dropping it. What is it called and how should I treat it?