I fractured my tailbone years ago and it has healed well, other than aching a bit in the winter . During my 5th pregnancy , my tailbone became agony to sit on, but this resolved itself after the baby was born. Now, I get a sharp pain if I sit slouched on something hard. Upon investigation, I have discovered a pea sized lump attached to the very base of my coccyx . It is not in the skin . I can push it under and up the inside of my coccyx, but not out towards the skin, nor can I move it to either side. I m 26, female, bmi of 19 and healthy otherwise.
First thing needed to be diagnosed is what the lump of?since it is diagnosed with some investigation, you need to discuss with your doctor about the type of lump.  You said you can push it. So it is not bone tumor. It seems a soft tissue lump. Any lump can be treated. There may be chances of a simple lipoma, or some benign or malignant tumor. Please consult the doctor in detail. Here without examination cant be said anything. I hope I answered your query. Wish you good health.