DD8 had just come to me feeling upset as she has found a hard lump under her left nipple. I've felt both sides and there's nothing under the right nipple. Google tells me it's a normal age for breast buds to start developing (I thought it was way too early for that) and that it often starts on one side and the other one "catches up" but I'm a bit nattered as I was a late developer and didn't get breast buds until I was about 13 or 14, and I (vividly!) remember the first one feeling like a little disk, whereas DDs is more like a small marble.
She has been quite emotional / moody lately so I had already been wondering if there were some early prepubescent hormones making themselves known, as she's always been such a happy little thing (nothing has changed at school or home).
Just after a quick sense check really as my first instinct is to call the GP first thing! Would you get it checked out immediately or keep an eye on it for a little longer and see if one appears on the other side?