It should go away in a day or two. Usually caused by a stiff muscle (not relaxing your arm) when they give you the injection. OTC pain relievers will probably work well. If you got the HPV vaccination, some people have pain for a longer period of time.
As a child, I lived in the Far East, and immunizations were passed out like candy. After getting ours, my mom always took us to the pool to swim. The excersize helped reduce the soreness...