Good thoughts, all.
Not blood glucose levels: we have a family history of diabetes, so I'm familiar with what to look for there. The only beverages I had this morning were caffeinated (tea, not coffee). Had breakfast.
My breathing was certainly off today: struggling for breath and stopping frequently as I did when I first began this whole bike-commuting thing. (Years ago I used to get cold-air/exercise-induced asthma, so maybe that's recurring. Never associated it with dizziness before, though.) I was on city streets, not the MUP, so I was exerting myself much more than usual.
Skullcap covers the top of my ears, but not necessarily the ear canal or the entire outer ear. Maybe earmuffs are called for.

Maybe it's all of this combined?