To me, it sounds like anxiety. You might want to ask a doctor about it though .
I would strongly suggest you have a complete physical and a very honest discussion with your PCP about your fears. I agree with the above poster that you most likely do have anxiety, but until any underlying medical conditions are ruled out, anxiety cannot be ruled in as your primary diagnosis. There are a number of issues that could produce the symptom you've described and those need to be considered first.
Is the feeling of not being able to breath your only symptom? Most of us with anxiety/panic have multiple symptoms, chief among them is a racing or pounding heart, and it is usually THAT which sends us frantically to the ER, but you say your heart rate is fine.
I stand by the suggestions you've been given to see your doctor. If it does turn out to be anxiety, there is much that can be done to help you overcome it.
I wish you the best
RubyWitch
To me it does., but I'm not doctor. Did you ask them? Call them up and find out what's up. See your general practitioner.