If it's more of a muscle pain across the backside of the shoulder you probably strained a muscle doing pull downs or back exercises.

Where is it on this diagram:


or this:



Assuming it's probably your infraspinatus muscle, which I was hoping for at the first, here is a little bit about preventative/corrective exercises:
http://www.exrx.net/WeightExercises/...lRotation.html

http://www.exrx.net/WeightExercises/...nclineRow.html

You will probably want to read the first section of this page:
http://www.exrx.net/ExInfo/Weaknesse...#anchor3549775