I think as anxious folk we are so very aware of any change in the rate our hearts beat at, or the pace at which we breathe that we pounce on it straightaway and wonder is it alright!!
Best way to deal with this one is to try regularly to get yourself slightly out of breath by doing a little exercise and then it will become 'ordinary' and no longer scary!!
I bought myself a little trampette/rebounder and when I first got it I would barely even stand on it [:I]. Nowadays I jog on it to music for 15 mins 5 days a week and now I don't even bother taking my pulse or staring at myself for signs of impending doom, plus my jeans fit better
If you haven't exercised for a long time start very gently - even a small walk round the block (if you can) adds up!!
Piglet
"Supposing a tree fell down, Pooh, when we were underneath it?" said Piglet.
"Supposing it didn't," said Pooh after careful thought.