Can i be pregnant?

I'm a nexplanon and I've been on it since July 6, 2017. I've recently had sex last week and ejaculation did occur and I've been freaking out bc I've seen on the internet some girls have gotten pregnant and I took a pregnacy test and it came out negative and I'm not sure if it's true or not due to woman who took a pregnacy and it came out negative but still ended up pregnant. I'm spotting sort of and having cramps. Should I be worried?