can I be pregnant? someone please help me :(

Okay so 3 days ago, I had sex and the condom broke when he came and he says he pulled out but some probably got in me or around my opening. I have nexplanon but I had it 12 days when this happened so about a 1 week and 5 days when this happened is how long I had the implant for. When I got the implant I was halfway on my birth control pills so I finished them I just didn't take them at the same time of night. But I am scared because this has never happened to me before. My doctor put it in so I assume it's right my friends just put it in my head that it may not be working and maybe I should get plan b... someone please answer me :( I am so scared. My last period was on the 21sr of last month and I still haven't gotten it...