It is a bit confusing! According to our obstetrician we’ll never know how it happened. Our fertility Dr said to keep trying the old fashioned way at the same time as getting the embryo transferred and we did, so maybe I got pregnant naturally, though we were told it was a 1 in 100 chance. They are not identical but apparently they may have still come from the same embryo (which I didn’t realise is a thing but apparently it is).
Here’s what the internet says:
“Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.”