After some research...Its the Infraspinatous muscle (right) that's strained...
The infraspinatous muscle outwardly rotates the humerus. Infra means below, spinatus means spine, referring to the ridge of the shoulder blade (scapula). If you touch your shoulder blade you will feel a transverse ridge running across it. Below this ridge is the infraspinatus muscle. It is commonly weak and underdeveloped. However, not everyone needs to roll the humerus outwardly; some people have it naturally outwardly rotated. This movement should only be performed until a central 'neutral' position is achieved, which can assessed by a qualified teacher. Overworking the infraspinatous results in unnecessary shoulder tension and pain.
Ashtanga Yoga, Practice & Philosophy ~by Gregor Maehle