Could be all kinds of things, so do consult with a doctor.
My dizzy spells can come from a variety of problems, but I'm familiar with all of 'em and can usually recognize the causes:
- Pinched nerve and artery in my neck from a damaged C2 vertebrae. No mistaking this one, because it's usually accompanied by severe pain and, if ignored, a spike in blood pressure. That's why I'll probably never be able to ride drop handlebars again, but can manage fairly upright bars.
The last time it happened was while riding was on a narrow bridge as another cyclist was approaching quickly. I turned my head too quickly to check traffic around me and pinched that nerve. As soon as I felt dizzy I swerved right to avoid colliding with the oncoming cyclist, and bumped along the bridge fence until I flopped into the grass on the other side. Embarrassing, but nobody was injured. I just laughed at myself for being a klutz.
- Migraine onset -- it's part of the aura the precedes a migraine. Not usually severe, but combined with the visual cues and other symptoms I know the drill. These happen infrequently as I've learned to avoid the triggers.
- Low blood sugar. As long as I eat properly and don't overdo it with sugary stuff I'm fine. I always take bananas to snack on during rides in case I bonk from blood sugar. These are predictable because I know when I haven't eaten properly.