Possibly supraspinatus tendinitis, but more likely it's supraspinatus impingement syndrome or subacromial impingement syndrome (same thing).
Best thing to do is to avoid anything that bothers it until you stop having any pain, then add some shoulder strengthening (nothing overhead and no upright rows) to your training. You can use ice as it is an inflammatory disorder.
I would lay off of any swimming that bothers it until it's better. It can turn into a very nagging chronic injury and eventually a rotator cuff tear if aggravated enough.
As far as adjusting your stroke, maybe reduce your reach distance, lower height during recovery phase, and avoid crossing over during the pull. Also, try to reduce the amount of shoulder internal rotation you are getting during the catch/high elbow portion.