Light headed post swim

Feeling lightheaded after swimming is pretty common. Lots of possibilities:

If OWS (or you are a very anxious swimmer), anxiety, which causes a lot of people to hyperventilate, which can lead to light-headedness.

Breathing issues in general, not just anxiety related but also due to poor technique, asthma, or poor ventilation. Many beginners don't exhale underwater completely, so they tend to try to breathe when their lung aren't really emptied from the previous breath. This can lead to feelings that one is short of breath while in fact one is hyperventilating. Results can include light-headedness. It's also possible with an experienced swimmer if (like me) one has asthma, and/or the pool is not well ventilated.

If the water was choppy or cold, could be vertigo. Cold water or chop can stimulate the nerves in the inner ear and cause a lot of people to feel light-headed or dizzy, especially when they go vertical. I'm a strong swimmer but in cold, choppy water, I typically get out, stand up, and fall over. Earplugs largely solve this issue.

If it was in a calm pool with normal temps and anxiety wasn't a factor, could be dehydration (yes, you do need to hydrate while you swim, especially if water and/or air temps are high). Also consider that you could have started the workout dehydrated from other training (if doing doubles) or just running around in the heat doing errands, etc.

Blood sugar levels. Maybe not enough calories in the system for the duration of the workout plus other training, work, etc. No need to eat "sugar", per se. Even a quick piece of sweet potato or fruit might give you a needed energy boost.

Finally, other medical issues ranging from minor (coming down with a cold/flu virus) to major. Not a doctor so not qualified to advise on that.

You might not be exhaling enough. Something about carbon dioxide buildup. Thats what I was told when I had that issue too. Your body will make sure you get enough air in your lungs to survive while you are swimming w/o having to think about it. You just need to focus on exhaling.

I used to get lightheaded when I swam, and for me it was a hydration issue. I take a bottle on the deck with me now, and grab a sip between intervals. I also found that plain water wasn't doing it for me, as I would then get headaches after my workouts. I add nuun to my bottles now, and am a happy camper.