Sex on cycle day 7 is not high risk for pregnancy. On average ovulation is on cycle day 14 and sperm only lasts for 5 days inside. It does not sound like fresh semen got in her vagina. I think you are safe. The bleed 7 days after taking emergency contraception in the first half of the cycle is normal and indicates the EC did its job. This bleed probably starts a new month, so do not get upset if her period does not show at the normal time. The back pain is probably due to the hormones in the pill.
I hope you are discussing proper means of contraception before engaging in greater intimacy.