What Causes Blue Nodule Under The Skin Of The Chest?

My 2 1/2 month old daughter has a blue nodule under the skin of her left upper chest. The area is about 1 x 1.5 cm and moves freely under the skin. It is soft, and doesn't seem to bother her when I touch it. What could this be? It has been there for 5 weeks now, and the pediatrician does not know what it is. We are being referred to a dermatologist. THank you.