Well it looks like you've followed the Next Choice guidelines, since it is supposed to be taken within 72 hours after unprotected sex.
If the medication tells you to expect your period to be late, then you should probably just sit tight and wait a few more days.
Also, like you said, tender breasts can be a symptom of the start of your menstrual cycle.
Since it has been a week since you had sex (and IF you are pregnant), you will most likely start having more symptoms (abdominal pain/cramping, frequent urination, spotting etc. Check out more symptoms here:
http://www.medhelp.org/symptomsearch)
If your period still hasn't come, it could be due to the fact that your stressing out too much, and it is a known fact that stress can alter your menstrual cycle!
Still, if your period is late, the best way to tell if you're pregnant would be to wait it out, look out for the symptoms, and do a pregnancy test.
Good luck!
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