HI,
Welcome to HCM,
I read your query and i appreciate your concern regarding this issue.
To be pregnant, 2 criteria should be fulfilled:
1. Fusion of sperm with ovum to form a fertilized ovum.
2.
Implantation of the fertilized ovum in the Uterus.
As per details You took
emergency contraceptive pill after intercourse after that you got withdrawal bleeding and then normal cyclic bleeding so there are no chances of pregnancy.
There is no need to worry about it.
Regarding your backache there can be other causes like abnormal
vaginal discharge in case of any infection.
For backache you can take any painkiller like
Acetaminophen 1000mg twice a day after meal.
If symptoms remains the same I would Advise you to visit Gynaecologist for proper examination and Pelvic
Ultrasound.
Hope so it will be helpful.
You can write again in case of any query or follow up.
Wish you good health.
Regards