What Causes Sudden Onset Of Muscle Spasm In The Left Shoulder?

I had my son in my arms and was bending slightly when I felt a sharp pain and a ripping sensation below my left shoulder close to the spine. I had trouble taking deep breath for few minutes, it now feels like I have a huge muscle spasm or contraction, it hurts when I get my back straight, when I try to lean toward the left and I cannot do any twisting motion. Should I wait couple days before going to see a MD