deltoid pain

Sometimes, after I throw, I get a pain in the lower part of my deltoid muscle (say in the area of most shots 1 1/2 to 2 inches below the shoulder). It doesn’t keep me from being able to throwing or seem to affect my speed; it’s just uncomfortable to a max of sharp localized pain. When I do arm ban exercises, it doesn’t happen as often or as sever.

Any ideas why this happens?

Doc says it’sjust a strain, no issues

It would be nice to know what caused the issue so you can avoid it in the future. Any idea?