Does it feel like you just having trouble accelerating and then the pain occurs once you do? If so, the racquet is either too stiff, too much weight in the handle - or could use use some lead at 12.00. I know it sounds weird saying add weight, but sometimes extra plough through makes it easier on your shoulder.
Yes, we know it's all about the technique. But if your racquet collapses every time you volley..you need to add some lead to it...