How Long It Takes To Cure Sore And Weak Arm?

Good day, I am a 45 year old female and in good health except for my left arm that I've notice is sore and weak for about 2 months now. It has gotten worse because now I am aware of the sore & weakness all the time. At first I though I had strained it lifting my heavy handbag, containing books, while in my car, that's when i'd use my left hand to get my bag from the passenger seat. Could I have hurt it that way and how long would it take to heal?