My fifteen year old daughter has a lump that moves in her left breast. It doesn't hurt. What could it be?

It could be time for you to take her to the doctor to get that checked out.

We can't diagnose this online and to be honest, this isn't something I would take lightly and just guess on...I would want to know form a trained professional if it is serious or not.

Hopefully a cyst but why worry? That's what Doctors are for.