Mild. For a week or so.
This is not unusual, it has occasionally happened to me for the last 3 years or so.
I haven't gone to the MD because I know what he is going to say (let it pass, it is normal) and what kind of investigative work he will do (none).
I am almost certain this has to do with my ears, since our body's equilibrium sensors are in our ears.
As a child, I suffered from repetitive ear infections. Basically, each time I went to swim, I had ear issues. My parents had to stop taking me to the swimming pool at some point. The MD's were basically clueless about the root cause, so the easy solution was to stop swimming.
This mild dizziness correlates with periods of high stress. I know, correlation does not imply causation, but it is a consistent data point.
Anyway. I'm not worried—for now—I think it will pass, in due time.