I am 13 and right now my breasts feel very hard. What's happening?
Like most of your body, your breasts are not made of just one thing. There are milk glands, fatty tissue to supply the milk glands, and the skin covering the breasts, just to name a few of the major parts. What is happening is that the internal parts are expanding rapidly as your breasts are developing, but they are going faster than the skin at the moment. Therefore, the skin on your chest is getting stretched and your breasts feel hard. Soon the skin will begin catching up and your breasts will feel a bit more normal.
There will also be a period when they will feel extra sensitive, this will come when the extra nerves develop around your nipples. This also will pass as you get use to the new feelings.