Hmmm...as a pretty severe asthmatic of 15 years (although not a doctor), I don't know if it was asthma or not. If it went away quickly, and you returned to normal breathing, it may have been a panic attack or perhaps a result of a combination of lots of things (you not feeling well, the hills, you getting dropped for a bit, heart beating rapidly going up that last hill). It may, however be exercised induced asthma. If it happens again when riding, bring it up to your doc (or bring it up anyway, and see what he/she says).

I'm glad you started breathing normally quickly. I know how scary it is feeling like you can't breathe. You take it easy, and take tomorrow off and rest- your body may be trying to tell you something.