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The lump or swelling on top of the butt crack is probably an abscess
or a cyst
.Please get examined by a doctor to find what it is and if it is an abscess then you will need antibiotics which the doctor will prescribe.If you don't get relief with the antibiotics or the swelling increases then you will have to consult a surgeon for surgical drainage
of pus if any or removal of the cyst if it is so.