What Causes Severe Pain In The Arm Post Flu Vaccine?

I received the flu shot 3 months ago. I expected a sore arm but the soreness got progressively worse to the point that I have limited motion in the arm. I can reach forward but can not elevate my arm raising from the side nor can I reach behind me without a muscle cramp from the shoulder to the elbow. I have an appointment with an orthopedic doctor this week. It is that painful and I have a high tolerance for pain.