Low blood pressure??

I would check with a doctor. Doubt it is low blood pressure. That is more likely AFTER the race, especially if you lie or sit down for a while and then suddenly stand back up. Yes, common in thinner, fit people, especially female endurance athletes, but not DURING the race. If the doctor doesn't find any issues, then most likely heat exhaustion, dehydration, overly ambitious pacing, or some unhappy combo thereof. Heat exhaustion in particular can cause the weird visual issues and lightheadedness you describe--I'm guessing your body is diverting blood from your brain in an effort to cool you off.

Low blood sugar is possible, but not likely in a sprint if you are otherwise healthy, and had decent food intake the day before and the morning of. It's probably not long enough to deplete you seriously, unless you have a tendency to hypoglycemia--plenty of athletes do sprints on zero calories, just water or electrolyte drinks, with no ill effects.

I would check with your local physician.

Thank you for your comments! I'll check with a doctor ...

First off...definitely...talk to your doctor.

But don't get all worried and worked up about it before you do talk to your doc. 'Cause it's not necessarily "low" blood pressure.
Just a rapid drop in blood pressure can do that. And that can happen if you're running...running...running...and charging through the last couple hundred meters to the finish line....and then you cross it and come to a dead stop while they put a medal around your neck and they start pulling the chip off your ankle and you're standing there looking around for family and where the food's at.

It'll happen to me. I've waved off the medal hangers and chip removers on occasion to trot around in circles to let my heartrate drop at a less precipitous clip which can lead to all that. The old "cooldown" that you probably normally do when you're just out for a run/etc.

The other time's I've had this is in a race where I was really pushing it on the run and then started to flag late (because I don't really know what I'm doing)....and went from a really good pace for me, to almost walking. Same result. High heartrate, all the sudden becomes low heartrate and BP drops quickly.

That being said, still talk to your doctor, but at least evaluate the context of the possible potential rapid change of HR and BP that may be a part of the equation. Or may not. But it's rather common enough that if that's the case, that should be part of your discussion.

I would ask your DOCTOR about Orthostatic Hypotension. I have had athletes that suffer from this as they get fit to the point that after sitting for an extended period of time they need to flex their quads to avoid getting light headed or syncope.