Fibrocystic Changes: This is the most common cause of breast lumps in young women. The presence of a tender cyst that has been there for 2 months is consistent with fibrocystic changes, which are influenced by hormonal fluctuations during the menstrual cycle.

Breast Abscess: Although less common in this age group without a history of lactation, a breast abscess could present as a tender cyst. However, it is usually accompanied by signs of infection such as redness, warmth, and fever.
Simple Breast Cyst: These are fluid-filled sacs that are very common in women, especially those in their 20s. They can be tender, especially just before menstruation, and can fluctuate in size.
Phyllodes Tumor: Although rare, phyllodes tumors can present as a rapidly growing breast mass. They are usually not tender but can be in some cases.

Breast Cancer: Although extremely rare in an 18-year-old, breast cancer must be considered, especially if the lump is hard, fixed, and accompanied by other symptoms like nipple discharge or skin changes.
Inflammatory Breast Cancer: This aggressive form of breast cancer can mimic infection, presenting with redness, swelling, and tenderness, but it is very rare and usually occurs in older women.

Hamartoma: A benign breast tumor that can present as a lump. It's rare and usually not tender.
Galactocele: A milk-filled cyst that occurs in lactating women, which would be unusual in an 18-year-old who is not breastfeeding.
Fat Necrosis: This can cause a tender lump but is more common after trauma to the breast.
Papillary Thyroid Carcinoma Metastasis to the Breast: Extremely rare, but can present as a cystic lesion in the breast.