Probably either enviromental asthma or you are way out of shape. Seeing that you live in CA, it could very well be seasonal, brought on by bad air. Happens a lot in cities with smog alerts. Could be fall allergies, lot of pollen floating around this time of year. A good allergist can help eliminate the obvious and pin down the culprit.
If its none of the above, then you need to work on your aerobic conditioning. 5 minutes to breathing problems is not a good sign. You are approaching couch potato levels, which seems hard to believe since you are on these boards.