I'm sure a few people here have had some experience with weight training related shoulder injuries... My shoulder's been somewhat screwed since my swimming days but this might be a little more serious.
On chest days, I can feel pain in the back half of the left shoulder usually after the third pressing excercise.
And on a back day, I get this popping sensation (not painful though) in the same shoulder when I'm doing pull-ups. It feels a lot like cracking your knuckles.. only I don't know if your shoulders are supposed to do that .
Sound familiar to anyone?