My baby is 9 months old now. i just found out i have low ferritian, never had before. i looked a recent bloodwork. did the pregnancy cause this.

Low ferritin: yes. Pregnancy can use a lot of your iron storage in your liver, especially if you don't eat food with higher in content, like meats and green vegetables

Combination : This is caused from a combination of factors, including both pregnancy, childbirth, and possibly breastfeeding. Think of all your body has been through! In growing this baby itself, the expansion of the uterus, creation of placenta and all the supporting structures of pregnancy you used up your iron stores, even with taking prenatals. Microcytic anemia associated? Prenatals? Best of luck!