There are two possible explanations in a case like this. The first is that your shoulder pain could be caused by your heart disease. It's not uncommon for pain from the heart to get referred to the neck, jaw, upper back, shoulder, and/or down the arm.

The nitric oxide (NO) in the nitroglycerin (GTN) opens the blood vessels so more blood reaches the heart. If the shoulder pain was caused by lack of oxygen to the heart, then the nitroglycerin patch appears to help the shoulder.

The second possibility is that NO helps damaged tissue heal. If you have a true tendon problem, the NO may be helping tissue grow and remodel. A recent study in Australia actually used nitroglycerin patches on the shoulders of patients with chronic shoulder tendon pain and problems. They compared the results to patients who got a dummy (placebo)patch.

The group with the real patch got much better results. Their range of motion and function improved much more than in the placebo group.

Be sure and report this incident to your doctor. It's important to find out if the shoulder pain is coming from the heart or indeed from inside the shoulder.