Chest/rib injury

One evening two weeks ago after work as I was sitting on the couch I started hurting in my left rib area and over the next few hours had difficulty breathing, as it was getting extremely painful to breathe, like my muscles were injured.
The next day, I had a swelling the size of a fist over my lower left rib cage and the pain went all the way around to my back and under the shoulder blade.
I continued to be in pain to the point that I could not wear a bra for a week.
I couldn’t stand the pressure of it on my chest muscles.
I have been that way now for two weeks but I have improved in the last 5 days – the turning point towards recovery being last Thursday.
However, I still have a slight swelling in the same spot and I have this feeling like someone is sitting on my chest.
Sitting for long periods of time seems to aggravate it and walking seems to relive it.
I would prefer to treat and care for myself and work with my natural healing processes, as I was raised NOT to go to the doctor (unless you were near death).
Would anyone venture a guess as to what I may have done to myself?
I was thinking that maybe I wrenched something carrying my laptop computer during the day, but I don’t remember doing so.
I have to carry one every day.