I had one postpartum also. I had low iron, which was borderline even with taking iron through out my pregnancy. But I also hemorrhaged badly after birth as well. I had three units of blood, and later had iron too. It didn't hurt per se, it felt strange, pretty similar to getting a blood transfusion. It helped me slowly feel better and after a week or so I felt almost energetic! But I also had to continue with getting iron shots afterwards because my levels continued to be quite low. I had those every week for 6 months, and they hurt like hell!! I still had bruises on my butt for months after having them, and the spots they went into my muscles were still tender for a long time after too.
It was 15 years ago now (after having my first angel), so I can't remember any of my numbers, but I do remember feeling quite energetic after having it after a week or so, though I think that might have been a combination of not being pregnant anymore, as well as having had a blood transfusion and iron.