I get it sometimes right after a long ride. I assumed it was due to dehydration. If you find different or a fix post an update.

I get the odd bout of the dizzies if I stand too quickly after longer rides (say 4-5 hours) - it's pretty much always dehydration related in my case - it's tough in summer to actually carry sufficient liquids to keep properly balanced, and on a solo ride, there very often isn't anywhere safe to leave the bike outside whilst nipping in to purchase a couple of litres of water somewhere. If I wasn't riding on my own, at least I'd have someone to watch the bike while I raided the shops. Outdoor taps aren't much of a feature in his country... So, very often I'll get home having done 130-160km on 1.5L of water - way, WAY too little, and in combination with relatively low normal BP and a RHR in the mid 40's, it's easy to stand up "between beats" and go a little light in the head...

spent a night in hospital last year after passing out after a big meal - 4 or 5 hours after a ride. I try and stay hydrated and have started adding electrolytes to my water. My HR is quite high on my rides, probably doesn't help - but it's hilly round here!

My cardiologist had me increase my sodium intake and also to use electrolyte tabs in my water when I ride.