In my experience with health anxiety, we can tend to focus in on what really concerns us and validate our symptoms as the cause of something much worse than it really is. You did the right thing by going to the doctor, and he/she said you are ok...which is great. Have you had anxiety issues in the past? Do you think quitting smoking will help your anxiety with this in the long term?
I can understand your concern since you are a smoker. But youth is on your side, you need to stop smoking to ensure that you live a long healthy life. I have a sister-in-law with Emphysema from smoking, gasping for every breath as she is dying from many years of smoking. There are so many aids to help you stop, no reason you can't do this. The shortness of breath can be caused by anxiety which the constant worry is causing. It does sound like you've pulled some muscles, try resting for a while and see if it gets better. Watch this anxiety and start working on it now, because it can become bad. I think the "rubber band' therapy would work well with you. Keep a rubber band on your wrist and every time you start to worry about this, snap the rubber band hard. It will sting, but the stinging stops your thought process. At first you're snapppig it all the time, but after a week or two you'll notice you're not having to do it as much. Many therapist use this with their patients. At first I thought it was "hokey" but had to try something, and it worked wonders with my worrying. It's worth a shot, and may help.
Take care....