What Causes Dizziness And Fainting Spell?

I am 42 years of age approx.164cm tall Female and weigh 72kg. My health and fitness level would be considered to be above average.
Recently I went for a short bike ride (approx.7 Klm) with my partner and towards the end of the ride I had to dismount from my bike and basically collapsed on the ground. I was totally fatigued, very light headed with zero energy physically.....even to sit or stand. My partner propped me against a fence in a sitting position....he did it all I had no power/energy to help myself.......I was tending towards having a "black-out". He gave some water and shaded me from the sun, it wasn't a hot day, just pleasant.
This is the second time it has happened over a period of two years...both times when I was bike riding.
Is this suggesting a blood/sugar issue or something more sinister?
Kind regards
Karen