left mid back pain

I was a frequent smoker (1-2 packs/day) for about 5 yrs and now (as of july of this year) i am a very light smoker (1-2 smokes/day somedays none) but i have been getting stinging and sometimes radiating dull mid-left back pains at random. I have seen a few doctors and have been told its nothing to worry about but its just uncomfortable not knowing whats causing it and the pain too. I have been told stress but im a very happy guy.
i am a healthy male aged 23.