Lump on baby

(First I am absolutely calling my pediatrician first thing in the morning but I guess I'm looking for some peace of mind/wondering if anyone else had this issue.)

My daughter is 4 weeks and 4 days old, she is primarily/90% breastfed and earlier tonight my mom noticed a little lump about the size of a dime on my little ones left breast area, under the skin kind of between her nipple and armpit. It didn't seem to cause her pain, it's not really hard but it is noticeable to the touch. There is no lump on her right side.