What is Lump on back heel only hurts when shoe rubs it?

Plantar fibroma:

A plantar fibroma is a small, fibrous lump that grows on the bottom of the foot, typically in the region of the heel. It can be uncomfortable or painful, especially when pressure is applied to it, such as when wearing shoes. Plantar fibromas are often noncancerous and can be treated with conservative methods, such as padding or arch supports, which can help alleviate discomfort and pain. However, if the discomfort is severe, surgery may be an option to remove the fibroma

Haglund's deformity (pump bump):

Haglund's deformity, also known as pump bump, occurs when the bony prominence at the back of the heel (the calcaneus) becomes enlarged and rubs against the shoe, causing irritation, pain, and swelling. It commonly occurs due to prolonged wearing of shoes that are too tight or have a stiff heel counter, causing chronic friction and pressure. Symptoms may include pain and tenderness in the heel, redness and swelling, and a hard, bony lump that can be felt at the back of the heel. Treatment usually involves conservative measures such as shoe modifications, cushioning, and physical therapy. In some cases, surgery may be necessary if conservative treatments are ineffective.

Retrocalcaneal bursitis:

Retrocalcaneal bursitis is an inflammation of the bursa, which is a fluid-filled sac that provides cushioning and reduces friction between tendons, muscles, and bones. When the bursa becomes inflamed due to excessive pressure or irritation, it can cause pain and swelling in the back of the heel, especially when pressure is applied to the area. Symptoms may include pain when walking or running, stiffness in the heel, and tenderness to touch. Treatment typically involves rest, ice, compression, and elevation (RICE), along with pain relievers and physical therapy. If symptoms persist, aspiration or surgery may be necessary.