Blackout cause???

I'll try and be brief... I went for a ride on Sunday in the afternoon, I'm 47, in good shape, it was about 73 degrees and kind of muggy. I was just out for a casual ride, didn't take water, wore shorts and a cotton tee. The trip was only 5 miles, 30 minutes, some hills and I was sweating pretty good - I've done much harder obviously. After about 15 minutes, i started to feel queasy, drank a glass of gatorade and headed up the stairs to shower. I felt like I was going to blackout so I laid down with my knees up and fought it off after a minute or 2. I got up, walked into the bathroom and got the same thing - white as a ghost - down to 1 knee, now I feel like maybe I got heat stroke but i wasn't out that long. So I got in the shower and nearly lost it there - tunnel vision, loss of hearing, fighting for air, struggling to stay conscious. I shut the water off sat down/ collapsed and after a few minutes i started to feel under control, blood back in my face, vision cleared, hearng back and was fine the rest of the day.

I'm trying to figure out what would cause it, I hadn't eaten for a few hours, no water but a glass of gatorade. Was it low blood sugar? dehydration? heat stroke? This was not a century ride by any means, I've been hotter, sweatier, gone further faster, pushed harder, etc. It kind of has me freaked out now. Has anyone else had this? No chest pain or left arm pain that would point to an obvious heart attack. I'm leaning toward heat stroke/ dehydration because of the gatorade (although a delayed reaction) and the shower to cool me down.