Pain (ache) under left shoulder blade and under left breast. gets worse with deep breath. can't inhale fully. is this sign of disease or muscle issue?

Possibly : Yes, it is likely a sign of muscle strain/spasm. But, there are also other potential causes. If you've done recent heavy lifting/strenuous exercise, had a cold wherein you coughed a lot, etc, then this is very much possible. If you are having any chest pain, shortness of breath, or fever, however, you should see a doctor ASAP. Best wishes.