C.P.
Its been awhile since my DD is 21 months old, but I remember having that switch in BM's too. I read that it was most likely due to changes in my diet. Some things we eat/drink just come out differently on the other end. Also her digestive tract is maturing, working a little differently, getting more out of each feeding. Consider moving up to the next size in diapers to help deal w/ the explosions.