calendar method

Is it safe to have unprotected sex ten days after a period?

Anonymous
asks:
I had unprotected sex with my girlfriend 2 days after her periods was over, as she told me that 10 days after period are safe to have unprotected sex. Her last period was 27th October. Well today, 19th November, she called me and said that her period repeats in 20-25 days and she should have period now but she is not having. Can you tell me the reason?...