hello,
my nephew had that!! worrying i know!! but we found out it was the material off a new top he had been bought, was rubbing on his nipples and made them sore, ( something to think about)
but in a more realistic way, if i remember rightly i developed very early, it is possible that she may start to grow very slowly over the next few years, i remember having boobs at 9! not sure about 7.
i also have a 7 year old and i to wonder about when and what to tell her, tho the simple fact is, each child is different, if you feel she is mature enough to understand about growing up and periods, you could start to expalin it in a way she will understand, i cant see that it will do any damage, and if she is already experiencing pubity, maybe the sooner the better. i havent had to explain much to my daughter yet, she hasnt really asked many questions, tho i will make sure i do explain it all properly in the next few years before she hears bits and bobs from school ! and pieces it together wrong like i did hehe
hope this helps xx