Being that your daughter is 12, is she fully developed yet? Is the lump on just one side or two? I remember when my daughter was about 11 or 12, we too felt a lump just underneath her nipple on one side. She said it was tender and painful. It turned out to be her breast "bud", the beginnings of getting breast tissue! Within a week, her other side began with the same thing. I think it sounds like your daughter is beginning to develop. Check with her pediatrician though just to put your mind at ease. Her doctor can give her a world of information on what to expect during puberty. Good luck and thanks for being an observant, concerned mom!!