Breathing bad but measurements are fine - what could it be?

[francisaugusto]

I have been an asthmatic since 2010, and had only a few crisis since then. I have been active, ran 13 marathons since then, and overall asthma hasn't been a bigger issue. I control it with Symbicort twice a day and, sometimes, but rarely, Ventolin.
Last year, I felt that I wasn't breathing my best, and the doctor told me that I should take it easy on my runs and gave me this device (PFM - Peak Flow meter) where I blow to measure how good I am breathing. The numbers were bad, but then they started to be good. He also meant it could be polution. I resumed running, and things were fine. This was around august. Now, since february, I noticed I started again with the sensation that my breathing is not ok.
Today I decided to give that device another run, and actually the numbers were even better than when I got better last year.
So what could be this sensation? Could it be the air quality? Just to clarify: it seems that my lung capacity if fine, at least according to PFM. But still, I feel this sensation I'm not breathing well...