Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concerns.. As per your complain a sore lump on tailbone can be due to most commonly formation of a Pilonidal cyst that is seen to be commonly caused due to either an ingrown hair or traumatic injury in the lower tailbone area.. I would suggest you to consult a General Surgeon and get evaluated and he can commonly clinically diagnose the lesion while can also advise blood test for confirming.. Treatment that can be advised you is commonly surgical by Incision and Drainage of the lump.. Marsupialization can also be advised to you. As of now you can sit in a tub of warm water to reduce pain and shrink the lump.. Take Ibuprofen to relieve pain.. Put gentle pressure to drain off the fluid from the lump.. Apply antiseptic solution to prevent spread of infection.. Hope this information helps.. Thanks and regards. Dr.Honey Nandwani Arora.