It's nothing to be too concerned about. Yes your breasts are starting to develop, starting with your nipples. The hard structure you feel underneath your nipples are the development of the milk glands that will become larger. Your soreness comes from the expansion of the areola and nipple area.
The soreness will become less over the next few weeks. Though your nipples will become much more sensitive and any brushing against them, such as through doorways, can be painful.
Welcome to the club, dear!
-Sandy