I have a small firm bb sized lump under my skin, what could this be?

More info- ? cyst: Although it would be useful to know more about this- where is it, how long has it been there, does it hurt, is there redness in the area, etc. That said, it may be a sebaceous cyst, which is quite common, and presents as a small lump under the skin. A cyst is not painful unless it becomes infected. I recommend you see your doctor or your dermatologist for definitive diagnosis.