What is a hard growing large lump on my tail bone? i have a large hard lump growing and changing shape on my tail bone and it is causing a sore to appear outside on the surface between my buttocks because it is protruding enough that it makes an indent on

You : You could have a perianal abcess. I know you don't have medical coverage, but this really does need to be evaluated.

The : The lump that you are describing may be a pilonidal abscess. These are very common in teens and young adults and are believed to arise from hair in this area getting under the skin, creating chronic inflammation. If this is an abscess, it will require drainage: i advise you to seek out medical care for this problem as soon as possible.