very solid, hard bump under the skin of my lower back?

Hi,
Welcome to the ehealthforum. I am glad to help you out. From the description of the symptoms, it can be normal coccyx that you have felt of a small cyst or swollen lymph node. Exact diagnosis can only be made by an examination, CT scan and maybe a biopsy of the area.

Lymphadenopathy (swollen lymph nodes) is usually caused by a viral or bacterial infection. Other causes include allergic reactions, arthritis, cancer, and an overactive thyroid (hyperthyroidism). I suggest you to get this lump examined from a physician.

It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.