Shoulder Pain, Impingement?

About a month ago, I started to get sharp pain around the head of the humerus on my left shoulder. The pain is greatest when:

1) Shoulder is abducted to 90 degress, elbow flexed at 90 degrees, and I push my arms above my head. An example being doing a dumbell shoulder press

2) When I internally rotate my arm - elbow flexed at 90 degrees and against my side, and bring hand to centre of body.

I went to see an osteopath, and he agreed that there could be some sort of impingement going on, and gave me a few exercises. I haven't seen any improvement though, so am looking for some second opinions.

As a bit of history, I am very into triathlons, and have swum competitively since a young boy. My ROM on external rotation of shoulder with elbow tucked into side is probably about 30 degrees, and it feels really tight. When I try to stretch in that plane, I get pain around the mid deltoid area.

Can someone suggest some strengthening and stretching Rx that I could be doing for this? Thanks