Metformin is not going to cause hypoglycemia (or at least to those levels). My guess is that the meter malfunctioned and caused a false reading.
Even if you were taking a sulfonylurea (Gliclazide) it would still be incredibly unlikely for it to cause levels that low. Heck, it would even take an incredibly rare and severe case of reactive hypoglycemia to actually reach levels that low.
If it were me, I'd chalk it up to a technical glitch.