It is definitely safe to continue breastfeeding while you are pregnant, but, because your body will produce less milk while you are pregnant, nursing could be more difficult. There are ways around this though. For instance, stay well hydrated during your pregnancy by drinking plenty of liquids to keep up your milk supply. Another reason to drink plenty of fluids is because breastfeeding can dehydrate you, which is dangerous for the baby you are pregnant with.
Breastfeeding also takes most of the calcium from your body, so it would be advisable to ask your doctor for calcium supplements to ensure that you get all the calcium you need while breastfeeding. This is not a problem for the baby you are carrying, however, as babies get all the necessary calcium from your body.
While it is possible to breastfeed during your pregnancy, it does take its toll on your body so you need extra rest and nutrition to stay healthy. So, take vitamins, eat a well-balanced diet and get plenty of rest.
You may experience some abdominal cramping while you are breastfeeding during your pregnancy. This could be because the nipple stimulation is causing your body to release a hormone that makes your uterus contract. If this happens regularly, then see your doctor for an examination.