Lumpy breasts can be caused by fibrocystic disease. This sounds worse than it is. It merely means that there is more fibrous tissue than fat tissue and that the breasts are "lumpy". It is rarely cancerous, but get checked alot just to make sure. I know whenever a lump occurs, breast cancer is suspected, and it's scary. Mammograms can be taken, but in a 19year old, whose breasts are still firm, this test is usually a waste of time because the breast is too dense. Ultrasound is being used more these days for younger people and men. Whatever is seen or not seen, expect a biopsy to be done on your daughter just because this test will be difinitive on a younger breast. A biopsy doesn't mean they "found" anything, but rather a precaution because they just want to make sure they aren't missing anything. Also, the breast is a sweat gland and as with all glands, may become inflammed and swell up causing a painful lump. I know this is a scary time, but there are other causes of painful lumps in the breast rather than cancer. Hang in there.
It could also be just a cyst, or estrogen or calcium deposits. You don't need to be afraid yet. If it is any of tese things, it is pretty much harmless.
Stop thinking malignancy. There are lots of causes for a painful lump.