After walking with my walker i feel a sharp pain under my shoulder blade on left side.... i do not smoke?

Left scapular pain: Tough one. Most benign cause muscle spasm (serratus anterior). More significant causes: cardiac issue, pulmonary embolism, aortic dissection, thoracic nerve root irritation, shingles, pleurisy, etc, try keeping your elbows closer to your sides as you walk, raise or lower the walker with your therapist, do wall push-ups etc and if no better or importantly if worse see your doctor

Muscle stress?: The extra work that your upper body has to do while using your walker may be putting too much stress on your arm and shoulder muscles. If you'll be using the walker for a long time, it would be well worth it to get physical therapy to increase your strength, endurance and flexibility so that you can both use your walker more comfortably and be less dependent on it. Best wishes!