Shoulder Pain / Symptom Checker @ The Mayo Clinic
The above link might help narrow things down, a bit, based on the symptoms you're experiencing.
If you've got swelling or serious discoloration or very limited range of motion, particularly if things don't improve in a few days, I'd certainly suggest speaking with your doctor to get a better evaluation.
Shoulders can be ugly little things, with what seems to be relatively mild injuries. Complicated connections, in there, and it's appalling how a seemingly little bit of swelling or bruising or muscle damage can often turn into something much worse. (All things considered, I'd much rather have a "charlie horse" or pulled hamstring.)