7 months pregnant and breasts have been leaking colostrum, third baby, nipples stinging really bad. why?

Body preparing: Congrats on baby number 3! your body has "been there, done that" before, and so knows a baby is soon to come and is now beginning to prepare for what happens after birth, including early milk production and toughing of the nipples. This time it recognizes the pattern, and so these type of changes occur earlier in subsequent pregnancies. Make sure you are getting plenty of rest to prepare for birth.