Breast lumps can be caused by:

1. Breast cancer
2. Breast cysts (which are fluid-filled sacs in breast tissue that are not cancer. The fluid in a cyst looks like water. An imaging test called ultrasound is used to find out if a breast lump is a cyst.)
3. Fibroadenoma (a solid, benign growth within the breast glands. It is a common type of breast lump.)
4. Fibrocystic breasts
5. Intraductal papilloma.
6. Lipoma (a slow-growing lump involving fatty breast tissue. It can feel doughy, and it is often harmless.)
7. Trauma to the breast from a bump, breast surgery or other reasons.

Breast lumps also can be caused by health problems that can happen during breastfeeding, such as:

1. Mastitis (an infection in breast tissue)
2. A milk-filled cyst that's usually harmless.