3.2 after a walk and then 6.2 ten minutes late? Sound about right. Usually takes me 10-15 minutes to get back up to normal levels when I see low values like this after a walk.
Or it was a faulty reading and you were already in the 4-7 range.
T2s on metformin don't need to eat to bring levels back up. At 3.2 there's probably no insulin being produced anyway, but glucogen will be getting produced to stimulate glycolysis/gluconeogenesis. If it goes low like this due to exercise just have a rest, and levels will naturally go back up. No need to start eating.