Pregnant on nexplanon

I have been in nexplanon which last for three years ,I have been in it for almost two years and my boyfriend and I have had sex on this birth control so many times .  My boyfriend and I haven't had sex in months and just last we had sex on Thursday and the week before on Tuesday and according to my boyfriend I told him two weeks ago that I feel like I was going to have my period and that was way before we has sex , now the last week the day after we had sex I started to feel nauseous and I started to see light spotting and over two days the spotting become more .