Is stiffness in thoracic area during movement a serious problem?

Hello doctor,

I had this stiffness, almost something pushing in my left thoracic area just under my shoulder blade, for the past six months and I feel it most when I am moving my left arm forward or keeping it forward or upward for some time (like for example when I wash plates). It is not painful, but annoying, though it stops when I move my hand down, or I hold my hand backward. It is only on one side, my left side. When I am sitting or lying, I am normal. I suspect an injury while doing free weights, cause this everyday annoyance came suddenly. I am being treated by a chiropractor for the last two months, who is slowly working out all my thoracic points (and says that I might have T6 and T10). What I would like to know is since this is a feeling present only at certain mechanical or postural movements, can it be anything more serious than a back injury (I have slight scoliosis and my shoulders are a bit arched)? How many treatments should it take for the stiffness or pushing to go away completely? Do I need an MRI to exclude more serious health conditions? (right now I am quite worried and would like to avoid MRI). I have been feeling like this for past six months but the stiffness is getting less often now, so it is improving. But I still feel it when moving my left hand and sometimes when I am standing, and it gets worse towards the end of the day. I do around three sessions for 20 minutes of cross-trainer every day. It does not affect me when I am doing it, but could it be responsible for my stiffness? Otherwise, I feel extremely healthy, though during the last period I felt a little tired mentally with all the overthinking related to this injury, cause I am used to being perfectly healthy.