It started, actually, after a case of bronchitis
I had a few months back. It feels like my lungs never went back to normal after it. As soon as the weather started getting very hot, I haven't been able to breathe normally - even albuterol doesn't fully work. It gets rid of my wheezing, but I still feel like an elephant is sitting on my chest, and I have to strain to take in full, deep breaths.
I've had asthma for years. I smoke 2 packs of cigarettes a day (I know, I know, horrible.