Not sure where you are but the air quality has been BAD recently and anyone with asthma etc is noticing the breathing difficulties in my area.
Others who are not diagnosed with asthma are commenting on the tightness in their chests so I think it is affecting more than the usual crowd. Lack of ability to breathe properly may cause your heart to race (a bit?) to get oxygen around your body. Do you notice that you breathe more easily when you are inside an air conditioned place (like work, library etc)?
I agree with another poster that a call to your doctor is the best thing, if you have one who will listen. Would an aromatherapy massage help? Hot rock massage? Boost your B complex vitamins?