If your breathing tests and lung capacity is normal then i dont find any reason for you to fall short of breath while talking to someone.
May be you talk too fast and doesnt try to breathe in between long sentences while u talk. This is a normal thing with a lot of people however some feel aware by it and hence this feeling of breathlessness.
At the most if you want what you can do is get an arterial blood
gas and see if there is any problem with the oxygen