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After taking the emergency contraceptive
pills, usually one will get there periods on the expected date.
among the side effects mentioned, irregular period is also one of them. You might not be pregnant, your period might be delayed.
As you have mentioned mentioned you have performed the urine pregnancy
test and had turned negative.
Few points you should be aware are. 1. it should be done week after the missed period, done earlier it might be false negative. 2. it is advised to use early morning urine sample.
If you have done it early repeat the test. UPT kit are nearly 98% accurate. If it is done properly and comes negative chances of pregnancy is very slim.
Back ache is not the sign of pregnancy. If the test comes positive you need to go for ultrasound
abdomen to rule out any fetal abnormality as you have taken pills.
Hope i have provided the information you needed.