Late period with EC?

Last month, I had to take emergency contraception. I've heard that it can make your period late or early by a few days. I'm now on day 5 since missing my period... And I'm feeling no signs that it's going to start soon. I'm pretty sure I'm not pregnant. I've taken two tests, one the day before my period was supposed to come, and yesterday which was when my period was 4 days late. They were both very strong negatives. I guess my question is what is everyone else's experience with this? After how many days did your period start? Is this normal for it to be this late or what?