Just thought I'd add my .02 worth here.  Boys can also get these hard, also
quite tender, lumps just under nipple around 10 to 12 years of age.  My son
was quite the worried one about a year ago (he's now 12.5 years).
He had one first on one side and then later on the other.  Both disappeared
after a few months...and no breast development...!  I'll have to look up the
physiology sometime...

Cheers from very snowy Sudbury, Canada

Genevieve Courant RN, MSc IBCLC