Does anyone know anything about box turtles and reproducing?

My 15 year old female box turtle layed an egg this morning and never has before. She never goes near any other turtles so she is obviously not mating.When i picked it up and put it on the table to see what it was, it cracked and yolk and yucki stuff came out of it.. im confused
Answers:
Turtles do lay unfertilized eggs on occasion. It will still have a yolk and the surrounding albumin like the unfertilized chicken eggs that people eat. Female box turtles can store sperm for a few years from an earlier mating, I've read of storing it for up to five years. So if she has only been in your possession for less then 5 years there is still is an outside chance of fertility.