[tanysquirrel]

most likely it is the lactiferous ducts are being developed. those are the 'milk' tubes that connect to your nipple for excretion. when a woman lactates those lactiferous ducts swell with milk. the MER or milk ejection reflex is controlled by a chemical called oxytocin which relaxes the smooth muscle of the nipple,allowing the milk to flow. prolactin is the chemical which is responsible for the production of milk.

if you notice a yellowish substance being secreted from your nipple, that is most likely colostrum. this is the 'first milk' containing a lot of a vital antibiotics for the baby.