A cyst or a lipoma can cause a hard lump just below the elbow. Cysts are characterized as soft or hard, painless or painful, according to eMedicineHealth.
Cysts occur anywhere on the body. A health care provider is able to evaluate the lump to determine if it is indeed a cyst. Another possibility is a lipoma, which is sometimes referred to as a fatty tumor. This is a noncancerous lump formed from fatty tissue. A lipoma can form anywhere on the body, but it is most commonly found on the chest, abdomen, arms, neck, shoulders, back and thighs, according to Healthline.