6 yrs old with breast bud???

Hello!  My 6 yr old came to me tonight and said her nipple hurt.  I felt it and was shocked to feel a hard lump under it.  Other side is totally flat.  She is a tiny little girl and not over weight at all, only 48 lbs and 6 1/2 yrs old and very active.  What could this be?  I am very scared.  I did not go through pubetry early (period at 12).  Is this a breast bud at SIX?  Could she have bumped it?  Should it be looked at?  If it IS a breast bud does this mean precicious pubery or could she still go into pubery at a normal time?  Should I be looking for other signs? THANK YOU!