Pain in the tailbone is such a common medical problem that it even has its own name - coccydynia. Pain in the tailbone can be divided into two major types - traumatic and non-traumatic. Obviously, if you fall down or suffer a blow to the tailbone area, you may break or bruise
the bone or the surrounding tissue there and this is a common cause of traumatic tailbone pain. Nontraumatic tail bone pain, on the other hand is harder to figure out sometimes.
By far, the most common cause of nontraumatic tailbone pain is prolonged sitting. Interestingly, this pain can develop at any time. Therefore, even though you say that you have been sitting all day at work for a long time without any problems before, this is still, I think, the most likely cause of your current discomfort.
I would suggest that you go to see your primary care doctor
about this issue. They will be able to examine the painful area and confirm the diagnosis of, likely, nontraumatic tailbone pain. If so, they will offer several recommendations for treating the pain, which might include acquiring a specialized cushion for sitting on at work and also taking anti inflammatory medications.