Nexplanon- late period

I got the nexplanon about  a week and a half ago. I was supposed to get my period this past wed but it hasn't come yet :/ they did a pregnancy test before I got it but obviously it would have been to soon show up. They offered me plan B since we had been using withdrawal but I didn't get it because withdrawal has always worked fine for us. Well now I'm freaking out because I'm late and my youngest is only 5 months old :/ I don't feel pregnant but I'm paranoid. I have a leftover test but I'm scared to take it. I'm hoping I'm just late because of the BC. All of my previous pregnancies were planned, that would be my luck!! Lol. Anyone else ever have that happen?