Nooo...Shoulder

I am feeling a pain in the shoulder. Yes yes, I know there are millions of threads like this floating around, but I have to ask. How can I distinguish between a normal shoulder muscle sore and a shoulder injury? Are there circumstances in which the shoulder hurts but for completely normal reasons? My shoulder is having a dull sore, not a sharp pain. I happened last night. I was swinging my arm around with no apparent reason, and doing backstroking motions when I suddenly felt a pain on the top portion of my deltoid. If I were to draw a circle around the pain, the circle would face out and forward. When my arms are over my head, there's NO pain, but when my arms reach about 45 to 30 degrees to my body, the pain starts. The pain is mainly when I'm moving my arm. Right now, there's no pain anymore. I gave it a night and today, and currently there's no acute pain, but just a dull soreness much like an aching muscle. However, last night I sensed a bit of pain while moving in the 45 degree zone mentioned above, but when my arms were above my head or even above horizontal I couldn't feel a thing. When the arm's hurting, after a bit of rest it felt like it had been charged up with lactic acid and had that kind of sore feeling (my entire arm). Then it disappeared. Right now I can't feel any pain while moving my arms, except that very mild soreness. Oh yea, I sense no obvious weakness in the arm or inability to sleep on it. So...what's happening? Any ideas? Thanks a lot.