Breastfeeding is not a form of birth control. It's true that you may not have a period while breastfeeding but that doesn't mean that you are not ovulating. If you ovulate and you haven't a clue when, well, you sure can get pregnant, again!! Then what?
If you are at risk for preterm labor or have a luteal phase defect or a history of early pregnancy loss, stop breastfeeding immediately. The contractions that you experience while breastfeeding, whether you are aware of them or not, may trigger an early loss. You may be a lot more uncomfortable breastfeeding, if you are pregnant again, just because the breasts and nipples may be more sensitive, as they often are in that first trimester. It is often the first sign of another pregnancy.
It is safe to continue breastfeeding while pregnant again, if you have no contraindications but be aware that your baby may know that something is different. The milk itself may change in consistency, the taste may be different and the amount of milk can decrease. Your child may become more fussy or seem to want to nurse more often than before. Make sure to hydrate especially well and nutrition for you, your nursing child and developing fetus is of great concern.
Fatigue in early pregnancy is an issue, especially while breastfeeding another child and caring for that child too. It is, again, okay to continue breastfeeding but at what cost? Were you about to wean anyway? Sometimes as you decrease feedings, ovulation is more apt to occur and that may be why you are pregnant again. As supply continues to decrease and your child "senses" that the milk has changed, it is the perfect time to take a break and focus on the next addition to the family. Get prenatal care right away and continue with those prenatal vitamins.
If you do not want to be pregnant again, for whatever reason, BE AWARE: breastfeeding is not birth control, even if you are not having periods. Use birth control. And if you have been using a birth control method and you still feel pregnant, even while nursing, get to the doctor for reassurance or confirmation. It's okay to nurse two babies, but do you want to?