Does anyone know how to tell if an injury is serious or not based on the type of pain?

I have pain on exertion right at the base of my arm, really close to my shoulder. It isn't the kind of pain I associate with a serious injury, but I've had it for at least a month and it hasn't gone away. I haven't worked out because I'm waiting for it to heal.

Push-ups and bench press really aggervates it.