This sounds weird, and I can't find any information about it.

For the last couple of days, when I'm sitting or in the car, I find myself getting slightly breathless. No pain or distress or sweating, more as if I had forgotten to take a breath.

When I'm active, there's no problem. I can climb stairs as quickly as my young daughter and breathe perfectly well.

I'm a 51 year old woman with no heart complaints or any family history of heart problems. I've been active for most of my life, but in the last year, I've changed jobs and am not getting as much exercise as I used to.

You say you use to exercise a bit? Well if so then I'd say that your body got use to it it's having a hard time adjusting to not having to work as hard also you said it happens when your sitting? Not many people know this but when you sit down your ribs compress your chest slightly and can cause some discomfort, try hands behind your head it opens your lungs and lean slightly in the seat it helps my grandfather who has breathing issues

Okay, I'll try that. I'm doing a lot of writing these days, so it's possible I'm hunching over the keyboard.

Well, for what it's worth, I went to a doctor today, and she can't find a thing wrong. My heart sounds normal, my blood pressure is normal, my lungs are clear and normal. She's puzzled, says she's never come across anyone who is breathless at rest but not when exercising.

I'd find another Physician, preferably a specialist.

Update. Today I was at the chiropractors, and he felt along my back then said "What did you do to your ribs? How can you breath like that?"

So he did some adjustments, gave me some exercises to do, and I've been breathing normally ever since.

Could it be he angle you are sitting in, most people, esp as you get older and not as active, will find there are some sitting positions where you don't breath as well.

It is something we have to be very very careful with when you are sitting someone properly in wheel chairs for long term usage.

You can experiment with using a O2 meter to show your oxygen levels and sit at various positoins, and see if one or more gives you less O2 levels.

In standing positions, often their O2 levels increased.