What Could Cause Sudden Dizziness?

I went on a bike ride yesterday and I got off of my bike to cross at some traffic lights. I then started to feel very dizzy and I sat down. I had passed out a few months beforehand and I felt the same way. I drank some water and closed my eyes and when I opened them again everything went bright white. It wasn't spots, my whole vision was bright white. It didn't hurt my eyes, I could only see white though. I laid down and got my sister to call my family for help. I stayed where I was and got a lift home.I had eaten that morning and I had a drink before I left. It wasn't until I got off of the bike that I felt dizzy. I hadn't been travelling very fast either.I was fine for the rest of the day and nothing has happened since. I am just curious as to what happened. I thought it was going to just be like the faint from a few months back, but having bright white vision scared me.