Sorry to hear that you have recently experience what is obviously a Hypo. Metformin is one of the safe medications and isn't known to induce a Hypo, so it must therefore be down to something else.
Solely eating food that have high Gi/GL ratings can under certain instances result in what is known as Rebound or Reactive Hypoglycaemia, where having reached a peak blood glucose level, the blood/glucose level then rapidly drops in a short time down to a level that is below the safe lower level (Hypo) before slowly recovering again.
This can be worsened by anxiety, stress, illness, alcoholic consumption without adequate food and is generally avoided by eating the higher Gi/GL foods with lower Gi/GL food items so as to balance things out.
Triggering the Hypo might also be accelerated by a sudden change in temperature, ie: coming into a hot environment from the outside cold, whatever caused it, I think we'll never really know.
This is probably something that is a one off instance, but is worth mentioning to your doctor, and most probably nothing to get unduly worried over. Taking simple precautions, by ensuring that you are eating beforehand and carrying with you some sugar sweets as an emergency will do no harm, even if they are never needed.