Pregnancy and Nexplanon?

I am aware that it's "highly impossible" to get pregnant on Nexplanon but I'm starting to doubt it's effectiveness. My very close friend got pregnant on IUD and I've know other girls who have gotten pregnant with implants in. Me and my boyfriend had sex, about a month ago. I got my period last week and things were fine up until this week. I've been feeling extremely sick and having a lot of back pain and I experienced a morning sickness type situation that was similar to my first pregnancy. I then had *tmi* a brown tinted like CM, it was not like the end of a period, it seemed like implantation bleeding. When my friend got pregnant on IUD she had a lot of bleeding, but her daughter was healthy and fine. I don't want to go rushing to the store for a test, because internet searches lead me to different answers. There are many women who have gotten pregnant off of it, and many who haven't. What do you ladies think?