Upper Back Pain. 12 Thoracic Area.

I work at a parcel service, I have been there for a year with no problems. It is now peak season, and the work is getting heavy. Just this week I have been having a pain in my upper back just below my left shoulder blade, not too far left (still spinal area) its not a sharp life ruining pain. But its definetelty not letting me have fun in my daily activities. It goes away in the morning. But it makes it way back along as the day goes on. Ive only had it for about 3 days. What im mostly concerned about is, could it just be a soar, and go away after a week or so? Will it be stuck on me for a long time?

Tia