Hello,
As per your description it seems what you have in your back is a surgical scar from the fusion surgery you had. 
When a person has an injury, the body responds by repairing the damaged tissue, which creates scar tissue.
Scar tissue is a collection of cells and 
collagen that covers the site of the injury. People can develop scar tissue on their skin as the result of an injury, surgery, or acne. 
Other areas of the body can also develop scar tissue, such as the heart muscle after a 
heart attack.
Scar tissue can present in a variety of ways, including: A keloid is a raised, red-skin colored 
plaque of scar tissue that can form on tissues after an injury. 
Keloids often appear on the upper chest, shoulders, and upper back.
A 
hypertrophic scar is a more common form of scar tissue. People with hypertrophic 
scars may notice that they fade over time.
A contracture scar usually occurs in tissues that have had a 
burn injury. These scars can impair the movement of the affected area.
To get rid of a scar, one can opt for a Scar revision surgery which is a cosmetic procedure usually performed by a plastic surgeon.
But, one issue of concern is that your scar becomes sore. It can be due to many reasons but the most important factor is to rule out any infection.
So, I would suggest you get yourself examined to rule out any pathology. 
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr Kalpajit Banik, Emergency Medicine Specialist