Coccydynia
The coccyx is a triangular bone that consists of three to five fused segments. Coccydynia occurs when the coccyx or the surrounding tissue is damaged, causing pain and discomfort at the base on the spine, especially when seated. Coccydynia can be caused by an injury, childbirth, repetitive stress, poor posture, being over- or under-weight, or aging. Sometimes, there is no obvious cause. Symptoms include pain, tenderness, minor bruising, and difficulty standing after sitting.
Arthritis 
