I am a healthy 31 year old female. I have been experiencing left arm pain as well as shoulder pain and heaviness for the last week. It usually happens when I'm sitting at my desk at work. It did happen last night and bedtime and made it very difficult to sleep. When laying on my left side I did experience a mild headache that would go away when changing positions. I did recently have a tdap injection in the same arm. What could this be?