Hello! I Was On The Bus Today And I Felt

Hello! I was on the bus today and I felt a wave of dizziness. It was a somewhat warm day (18C) and I had drank a bit of water and eaten about 1-2hrs earlier. My vision slowly turned more and more white until I was blinded. My friend helped my sit down and I immediately started feeling better. My vision turned back to normal within 20 seconds and the dizziness faded after I got up and started walking around again.
Any idea on what this was and if it happens again what I should do about it?