showed carcinoma in situ with clean edges, which says that the cancer was removed completely. Still if it wasn't, then it would first present in vagina or uterus
and the symptoms would be abnormal bleeding or dirty discharge.
Pain in the tailbone occurs in many people who have slightly longer tailbones and sit on hard surface, because the tailbone presses against the hard surface.
In my opinion, this pain is not because of cancer spread.
Try using a soft mattress when you sit on something hard. You can do hot fomentation for pain relief. I'm sure the pain will be relieved. If it still persists, then you can consult a gynaecologist.
Hope I have answered your query.