[pud's friend]

I don't think it's an MS thing but maybe someone can help me out here.
The last few days I've been very breathless. No anxiety, just a bit puffed after the smallest exertion (e.g. getting dressed). I can liken it to how puffed I'd get when my thyroid was playing up (overactive) but without the palpatations, fast pulse or goitre.
It has been very dusty here over the last week, low pollen, no allergies.
Been on Beta for a year (!!!).
Stopped smoking one month (yeeeeaaahhhh). No cough or yucky stuff coming up.
Exercising as normal, alternate daily swim and lots of stinky, sweaty horses.

I'm thinking it's my lungs finding their feet again. Any ideas from some ex smokers?

Thanks, as always.