with ur description the pimple u r talking about can be sebaceous cyst . it is a type of epidermal cyst
.Sebaceous cyst are caused by a blockage of the sebaceous glands. Sebaceous glands secrete 'sebum', which lubricates the hair and skin. A sebaceous breast cyst
is a type of epidermal cyst, which forms as a collection of keratin-like material. Normally, a sebaceous breast cysts will have a small opening to the skin, but which may not be easily visible. Sebaceous cysts are most common on the face, back of the ears, and back, but also occur on the breast. Sebaceous breast cysts often appear as small bumps or lumps just beneath the skin. You can think of them as small, closed 'sacs', that contain keratin, which is a foul smelling 'pasty' buildup of protein.
Sebaceous cysts are caused by plugged ducts at the site of a hair follicle. Like a cyst, they move freely under the fingers. Hormone stimulation or injury
may cause them to enlarge. A sebaceous cyst that does not cause symptoms does not require medical treatment. Removal usually involves making a small incision in the skin and removing the entire sac so that it does not return.