What Causes Muscle Pain In Between Shoulder Blades And Back Area?

I have had what I initially thought was muscle pain in the center of my back since late October after carrying a heavy pumpkin on my right shoulder for about 20 minutes. No pain immediately, but after we got home I went to pick up the pumpkin and got a terrible sharp pain in between my shoulder blades, and immediately started having upper back and neck muscle type pain. I have been treated by a chiropractor for over two months with no improvement, as well as had two sessions of deep tissue massage which made me sore. Both times the therapist said that she could feel a knot just off to the left of the spine that didn t break down. My family doctor put me on steroids and muscle relaxers which have made no difference along with 4-sessions of physical therapy. I feel the pain more so when lunging forward or carrying even a gallon of milk. HELP!!!