I can understand how these changes may be concerning. I recommend you discuss them further with your obstetrician, as it is difficult to determine the cause without a fully medical history and physical exam.
During pregnancy, the mother's breasts and nipples undergo many changes in preparation for lactation and breast feeding. Due to the interactions of multiple hormones, including estrogen, progesterone, and prolactin, breast tissue increases, and milk production begins. The breasts can become engorged with milk and painful. In addition, the nipples typically enlarge and may change color due to changes hormone levels and blood flow. The nipple changes are also to facilitate breastfeeding. Also these changes may be normal, I recommend you discuss this further with your obstetrician.
Other dermatologic conditions can cause abnormal spots on nipples. A abnormal reaction to your clothing or laundry detergent, termed dermatitis, can cause inflammation in the skin around your nipple. It is possible for breaks in the nipple skin to lead to an infection. It is also possible these are small moles that are changing in shape or color, which could be indicative of skin cancer. Again, without a thorough evaluation in person, it is not possible to definitively say what is causing these changes, and I recommend you discuss these possibilities with your obstetrician.