symptoms? level of concern?

Hello all!
I'm so thankful for this forum.
I'm curious as to how concerned I should be and what my chances are of having lung cancer. I'm a 26 year old male. I started smoking at age 18. I quit a couple times for many months, but then I'd started again. I have officially quit smoking since March-ish. At the beginning of this year I had 2 very bad cases of the flu. I have never produced so much mucus in my life. I experienced wheezing, shortness of breath, coughing that lasted a month for both times I had the flu, and pain in my back when I inhaled deeply, which I think was due to the coughing. Like I said, since then I quit smoking. All those symptoms are gone and only lasted about 3 weeks to a month. Now, my concern is I have the smallest amount of pain in the middle, left, kind of upper area of my back. It's kind of similar to the pain that I had when I had the flu. It's in the same spot. It kind of feels like an ache/knot. I think I've had it for a couple of weeks now. It's not a constant feeling. It comes and goes, and is not disturbingly noticeable. It's not felt when I breathe deeply like when I had the flu. Nonetheless, I do feel its presence. It also kind of feels like it's deep inside of me. This is all I'm experiencing right now. My questions are as follows: Considering my background and age, how concerned should I be? Does this sound like symptoms of lung cancer? Should I do anything about it?
Thank you all in advance!