Hello,
It can be a boil or a pilonidal cyst. Pilonidal cyst is cyst right over the coccyx area. In this the skin becomes reddened or even sometimes looks like a small bruise. When it is infected, it causes pain. Numerous studies have been put forward advocating one treatment over another but none has been proved better than the others. Presently, the various methods used are incision and drainage,excision and healing by secondary intention, excision and primary closure and excision with reconstructive flap techniques. This is to be followed by antibiotics. Please discuss these options with your doctor.
I hope it helps. Take care and please do keep us posted in case you have any additional doubts. Kind regards.
oh and its the size of a pistachio