A hard lump that you find or feel under your skin could be a simple harmless skin cyst that is non cancerous.
Sometimes you can develop skin cysts under your skin that are closed pockets of tissue that fill with fluid.
The skin cysts will feel like large peas under the surface of your skin and although they can be annoying they are not harmful and do not require medical treatment and will usually go away on their own within a few weeks.
Skin cysts can sometimes occur as a result of infections or even clogging of sebaceous glands.
I sometimes get those hard lumps under my skin and I went to a doctor to find out what the hard lumps were and the doctor said they were harmless cysts.
If the cysts do not go away or you feel lumps under your skin especially breast lumps they could be breast cancer or other cancerous tumors developing so if in doubt go to a doctor to get checked out just in case.