It is unlikely that it is cancer, but it is wise to check it out.
Hi,
I don't think that this lump has anything to do with breast cancer.But it's a good thing that your mom is taking you to see your doctor.
It is quite common for teenagers to get lumps under the nipple and it may affect only one breast.This is due because of hormonal changes in the body as it develops.Usually these lumps take a few weeks to disappear providing that the area around the nipple doesn't look red, swollen and isn't causing any pain,because it could indicate an infection.
I am sure that the Doctor,after examining you, will reassure you that what you have is very common in teenager whose breasts are developing.
Take care ....