Emergency contraceptive pills like i-pill is usually taken within 72-hours of unprotected intercourse. It helps in prevention of pregnancy but has a failure rate resulting in conception even after taking the pill. Therefore, conceiving even after taking emergency contraceptive is not harmful for the lady and the child. Unnecessary fear on the mind of the users can lead to unwanted abortions.
As you are a breastfeeding mother, the demand of calcium in your body is high. The pain on your back is most commonly due to deficiency of calcium and vitamin D. It has nothing to do with the Cesarean delivery. Please start taking tablets for calcium and vitamin D. Consult your physiologist for spinal exercises. Consult your orthopedic doctor and you should be fine in a month or two.