What Causes Green Bowel Movement In An Infant?

I have 2months and 3 weeks child he pooh dark green for the past 3weeks since I have been changing formula (is formula feed only) first month i was using hipp organic pooh was fine and was pooh everyday (yellowish and texture and size) second month change of formula due to unavailability of stock i changed to S 26 he never pooh for 4 days then change to NAN H.A he pooh dark black 3days later I changed to novalac caesar (premium) the pooh now is dark green and small in size and pooh every day -is this normal if yes or no then why ? -is the baby having a bug (bacteria)