I Had An Unprotected Sex With My Girlfriend On 11th

I had an unprotected sex with my girlfriend on 11th to 13th this month. She was having a safe period, that's why we had unprotected sex. I cum her many times. she is still not having her periods, tommarow is the final day to get her periods. Suggest me some effective method to prevent her from getting pregnant. I'll be grateful if u kindly suggest me some pills. Thanking you.