My 11 yr old felt a hard lump on her lower breast. she says it's unmovable, hard, and is a bit painful when pressed on. what could it be?

See your doctor: What may be hard for her might not be what a doctor would describe as hard. The best thing is to get it checked. Hard, immobile lumps are never left alone. Get it checked out.

Location: If centered under the nipple, this is likely the early awakening of her breast development. It can start on one side before the other & the rapid growth stretches fibers & can cause pain to touch or when clothing rubs the area. If it is anywhere else, it would not be. Her regular doc could confirm or exclude the possibility.