What should I do if my baby is throwing up blood?

Call your doctor. You need to seek medical attention immediately. If your baby is breastfeeding, it's possible that your nipples are cracked and bleeding and your baby may be spitting up maternal blood, but there are other more serious areas of concern. Calling your pediatrician for this is the best advice i can give.
Call your doctor. In the first few days after birth, some babies may swallow blood from any cracking in the mother's breast. This is not uncommon, but can cause stomach upset. Other more serious conditions, such as severe milk allergy or infection can cause a baby to spit up blood. In general, it is best to contact your pediatrician to make sure that this is not a more serious type of problem.