The fetomaternal bleed must have been severe enough to cause a drop in hemoglobin
to 2.2 . It could have resulted from a traumatic delivery or an abruption but thankfully your baby recovered.
Since your baby was born preterm, you cant expect the baby to exhibit a blood profile similar to that of a term baby. All babies , whether term or preterm experience a drop in hemoglobin after their birth even if there are no complications. This drop in hemoglobin is seen at around 6-8 weeks of life and is known as physiologic anemia
of newborn. The drop in hemoglobin is more in case of a preterm and the range is 7-9 gm/dl and in term babies it is 9-11 gm/dl/.
So dont worry. Although your baby was well transfused at the time of discharge, this is an expected phenomenon.