hi. from personal experience of something similar I know that feeling of not getting enough air is terrifying. I used to get panic atatcks from it - massive rushes of adrenalin sending my heart racing, light headed ness from too much oxygen - feeling weird like gravity has changed from too much oxygen. Look into Hyperventilation Syndrome - basically bad breathing technique that means you over inflate your chest rather than make the diaphragm do the work. I had breathing retrainging and it helped a lot. The strange part for me was discovering that food allergies caused me to revert to chest breathing - I presume because gas bloating in the stomache made it harder fro the diaphragm to work against. I also noticed I held my abs really tight as a stress reaction which prevented preoper breathing. My food allergies (wheat, gluten, soy, dairy) also give me excess respiritory mucous and post nasal drip - it could be so much that it really felt like drowning in the stuff - I'd spit evey 10 seconds for an hour... Changing my diet to elimintate the allergens solved everything. Maybe you have food allergies or airborne allergies: dust, pollen, mold, pets... Maybe you can tie it down to a location or a time (dinner time having pasta was terrible for me). Good luck.
I know you said you are not taking any medications but did you have maybe surgery or any other incident where you might have been exposed to Hydrocodine, Vicodine, Roxicodone, OxyCottin, Morphine?
I kind of know the feeling you are describing just that I have it while in the shower/humidity. If yes I'd like to tell you more about what happened to me...
Allergies do not vary with either temperature or humidity enough to make the kind of difference you are experiencing. Pollution levels might change with air current. However it is unlikely for pollution levels to change with temperature changes. The pollution index in your area should be easy to track.
It is clear that this problem has to be evaluated when you are experiencing symptoms. Otherwise it will be difficult to determine if this is from your lungs, upper airways or heart. Any of the three can create the feeling of not being able to breathe.
I would go to an emergency room or your family doctor when you are experiencing these symptoms. They could do a thorough physical exam and a lung function test.
Have you changed elevations? If you've gone from a low to a high elevation, your body needs time to adjust and make more red cells to carry oxygen.
I live at around 620 feet above sea level. Years ago we went to visit relatives in Denver, Colorado, which is over 5,000 feet above sea level. The second day we were there they hauled us up to 10,000 feet above sea level. I could hardly breathe and couldn't walk very far. I couldn't really enjoy the view they wanted us to see because I was concentrating on just breathing. They didn't seem to have much problem with it as they were acclimated to the 5,000 foot elevation, and they were in better physical shape than I was.
It takes about 6-8 weeks for your body to replace red cells lost after donation. I would suspect that it might take you this long to adjust if you've changed elevations.
This is just a guess as to what's going on. Hopefully that is all that IS going on.