What causes the burning feeling you get in your throat and lungs when you exercise beyond your normal capacity?

Anyone with a magnifying glass or a shaving mirror can light a fire by focussing sunlight into a spot on a piece of paper and burn a hole.

Can the same trick be done with starlight? If so what diameter of mirror or lens will be needed? And which star would be the best choice?

Whenever I burn myself on the iron, the burnt area of skin goes darker and eventually peels off.

However, when I touched a hot pan, I got a blister instead.

The burns were caused through contact with hot metal, so why were the results different?

Eating a roast beef and mustard sandwich is one of life's simple pleasures* - except for when the mustard flavour seems to go up your nose to create a horrible burning sensation! 

I get the same effect from horseradish too - but nothing else causes this when I eat - it is unique to those two condiments...and can be almost painful!

Can anyone explain what is happening here?

* apologies to vegetarians - cheese and mustard/horseradish instead?