I don't remember it hurting much... Just my leg kinda went numb, but I think that was just my mind. But yes, it is a serum, so it hurts a little worse than a liquid shot (flu shot perfect example)
But yes, if the baby has blood (they do tests), they will give you another wonderful shot within 24? hours after delivery.
BTW I'm O-, so if you get a blood transfusion without them knowing your blood type, it's mine you get

Which makes me want to give blood all the more...