Carriel, no - the miscarriage won't be related. It is only in late pregnancy that breastfeeding may induce an early labour (in those at risk) and even then the baby(ies) would have to be feeding a lot. See this article below from the La Leche League site:
I'm Pregnant and Still Nursing My Toddler--Must I Wean Now?
Finding out you are pregnant does not mean you must stop nursing your toddler. Many mothers choose to continue breastfeeding throughout pregnancy, while others decide to wean. The following information may help you decide what is best for you and your family.
Relatives and friends may tell you breastfeeding while pregnant is dangerous. It may reassure them and you to know that, according to THE BREASTFEEDING ANSWER BOOK, "There is no documented danger to mother or fetus when mothers breastfeed through a healthy pregnancy" (Mohrbacher and Stock, 1991, p. 365). In a normal pregnancy, there is no evidence that continuing to breastfeed will increase your risk of miscarriage or deprive your unborn child of necessary nutrients. However, if you have a history of premature delivery or other risk factors, your physician may advise weaning as well as suggest restrictions on other activities.
Feeling tired is normal in pregnancy, but breastfeeding itself does not make you more tired. In fact, you may be able to get extra rest if you can convince your toddler to lie down to nurse. Many pregnant women report feeling restless or "antsy" while nursing an older child. You may find your nipples are sore. Careful attention to your child's position can help. Some mothers find breathing techniques from childbirth classes can help them cope with soreness. If your child is old enough, you can ask him or her to nurse more gently or for shorter periods of time.
Women who breastfeed while pregnant often find their milk supply decreases around the fourth or fifth month. It's also not uncommon for the flavor of your milk to change. These changes encourage many children to wean themselves. After the new baby is born, it's not unusual for the older child to want to taste the milk or ask to nurse again.
If you decide you want to wean your toddler, it's easiest to proceed gradually. The approach of "don't offer, don't refuse" is one that has worked for many mothers. See if you can anticipate when your child is likely to ask to nurse and offer a distraction or a healthful snack instead. Don't sit down in your favorite nursing chair. Be sure you give lots of extra hugs and attention. If your child has a strong need to suck, a bottle or pacifier may be appropriate. Remember that weaning a toddler can be hard work: continuing to nurse is sometimes easier for the mother.
If you continue nursing through the pregnancy, you may find yourself breastfeeding both an infant and an older sibling. Many mothers have found this arrangement, called "tandem nursing," is a good way of meeting the needs of both children. Your local LLL Leader can give you more information on breastfeeding while pregnant and/or tandem nursing. You can also find information in the LLLI publications below:
Mothering Your Nursing Toddler by Norma Jane Bumgarner. Franklin Park, Illinois: La Leche League International, 1982.
Nursing Two: Is It For You? Franklin Park, Illinois: La Leche League International, 1989. Publication No. 53