nexplannon symptoms

I have had nexplannon for about 2 months I wanna say and my boyfriend started cumming in me roughly 3 maybe 4 weeks ago. 
 I know I can still get pregnant like it wouldn't be the end of the world it's just preferred that I am not. 
Anyway I've had weird abdomal pains and been SO TIRED and nauseous and I always have headaches 
As I've heard it's so unlikely to get pregnant on it should I just not bother testing? 
As most people say the symptoms on the implant are really strong usually and are pregnancy symptoms so could it just be a new phase or soemthing idk.