What can cause sharp pain under the right shoulder blade?

Muscle Pain: There are some muscles that attach right on the shoulder blade called the rhomboid muscles and you may have strained one of those at the attachment. Usually we use these muscles when our arm is above our shoulder so perhaps some activity strained this. If pain doesn't resolve over next seven days or so you should get checked as sometimes the lungs can cause pain around the shoulder blade.

Referred Pain?: The same nerves that innervate the diaphragm go to the shoulder, as well. Therefore, anything that irritates the diaphragm, such as gallbladder or liver problems, can be "referred" to the shoulder. If the only symptom is shoulder pain (ie, no abdominal pain, nausea, vomiting, etc), i would search for other causes such as a muscle pull, arthritis.