You had your sex within 4 days of expected periods, which itself is a safe period if your menses cycle duration is of 28 or more days regularly.
Inspite of that you had i-pill within 72 hours of first intercourse, emergency contraception
has some side effects like causing irregular menses or spotting etc.
What you have experienced may be a adverse effect of ipill
There is almost nil chance of getting pregnant.
But in doubt you can have your urine pregnancy
test around 2 weeks from now.