13 year old has something hard under her left breast that was before painful bit not now, what could it be?

Likely breast tissue: You don't mention her puberty status so there are two common issues.As puberty begins, a spongy area of breast tissue centered just under the nipple on one or both sides begins to respond to hormones.Its rapid growth can stretch the tissue fibers & become sensitive. Gradual breast development follows. If much further along in breast development, nodules at this age can represent cysts.