Possible injury? What is left to question? 4 days later and you are still having trouble breathing, this sounds like a definate injury my friend.
I think a good general rule is to give something a week and if it is not improving, get it checked out. However, because this affects your breathing, you have a need to be more concerened.
Without knowing more, the first thing that pops into my mind is strained intercostals (the tissue connecting your ribs). A worst case scenerio is a spontaneous pnemothorax. Big word, but what it means is that air leaks into the space surrounding your lungs. Not likely that this is what you have, but if it is, this is serious.
If it were me, I'd go to the doctor as soon as possible. Than again, I am in the army and I get free medical coverage (you get what you pay for, and this is a medic saying it).
In the meantime, try icing the area, maybe taking ibuprofen or other NSAIDS, and avoid strenous activity.
Good luck, keep me posted, I hope everything is well and if nothing else professional curiousity.