What could cause tightness/pain in central left back (area appears to be swollen too) - it has lasted almost a week.?

Muscle Strain. A muscle strain may lead 2 pain and swelling. A specific cause is scapulothoracic dysfunction. It could also be a benign fat tumor called lipoma. More likely if this had not come to your attention until you bumped or hit it. Could also be a cyst that has become infected or inflamed under the skin. My best bet is a muscle strain. If it is red hot or swollen, it could be a an infection.