Hi, I went through the same thing.
Peak flow meter measures the openness of large airways.
When you feel breathless but your peak flow is relatively high,
only your small airways could be getting narrower.
To confirm this, you want to take a pulmonary function test when you have the feeling, which objectively shows whether your small airways are obstructed.
Also, I personally think that worsening asthma and mood swings, such as anxiety and irritability, go hand in hand. My doc says that changing hormonal balances in my body can cause mood swings, particularly when my asthma is getting worse. To deal with this, I'm trying to relax myself by drinking herb tea, taking nutritional supplements, doing Yoga and doing GSR biofeedback etc. In addition to these things, I often read books on psychology to deal with stress and anxiety.
Anyway, I think that you can use your feeling as an early warning signal of worsening asthma. Or if it's just an imagination, relaxation techniques I mentioned above might help. Hope this would help.