It is normal for the breasts to become heavy and uncomfortable when the milk transitions from Colostrum to Mature Milk. The body still does not know how much milk is required by the baby and tends to overproduce. You can expect this to happen between 2-6 days but it rarely lasts longer than 24 to 48 hours. If the breast becomes very heavy then it may become difficult for the baby to latch and this can cause breast feeding concerns hence it is important to take care of the situation.
How can I prevent or reduce engorgement ?
Before you begin nursing the baby, gently massage the breast and apply a warm compress. Once you finish nursing if the breast still feels heavy you can apply a cold compress and express a little to give yourself comfort. Remember, do not over express as this will send a wrong signal to your body. While feeding the baby you can gently massage the breasts to encourage effective emptying of all the ducts.
If you find that you have developed a fever, are having body aches, if your breast appears red and painful then do consult a lactation specialist as this could indicate a breast infection.