Did Next Choice EC Work?
Since we are unsure when in your cycle the Nexplanon was placed, here is some basic information on how it works. Nexplanon starts working right away if it is placed within 5 days after the start of your period. If you had Nexplanon placed at any other time during your menstrual cycle, it would be necessary to use another method of birth control, like condoms, for at least 7 days to prevent pregnancy.
After you take emergency contraception (EC) it is normal for your next period to be different than it usually is. It may come earlier or later than it normally does. It is also possible for it to be heavier, lighter, spottier or the same as usual. So, what you are experiencing could certainly be your period that is just presenting a little differently than it normally does because of taking EC.
You mentioned taking Next Choice Emergency Contraceptive the same day the unprotected sex occurred, however, it is only about 89% effective in preventing pregnancy when taken within 24 hours of unprotected sex.
Unfortunately, we cannot diagnose over the web and even though you mention having a negative pregnancy test, the only way to know if it is accurate is for you to take another pregnancy test and/or make an appointment with the medical professional who implanted the Nexplanon.
TeenHealthFX recommends that all sexually active teenagers use condoms every time they have sexual intercourse. Condoms are the only form of birth control that may protect from sexually transmitted infections and should be used in addition to another form of birth control for best protection.
If you don't have a doctor and live in northern New Jersey, you can call the Adolescent/Young Adult Center for Health at 973-971-5199 to schedule an appointment with an adolescent medicine specialist.