Hi, if you are pregnant, the little life inside of you will be needing a lot of nutrients from you. At the same time if you are also (tandem) breastfeeding your 11 month old will also be needing a lot of nutrients for you. If you dont't eat well enough, you might be too weak to carry the pregnancy, and the milk that you feed your 11 month old will be not as nutrient rich as before.
Personally I decided to stop BFing totally once I found out I was pregnant. I told myself that if I do wish to give my girl breastmilk again, I can always wait until after I give birth.