I've had the Nexplanon BirthControl in my arm for about a month now, I've had my period for the last 2 weeks nonstop, but it's not a normal period blood, it's like a thick, brownish color blood. Should I be concerned? Should I see my doctor?

That is old blood, which is odd. I would see your doctor about this. You got a bunch of literature with that, did you read it over. A lot of times it will mention side effects and when you should get them checked out.

What do you mean old blood?
Like I don't understand what's going on, I'm 17 . Is there a chance I could be pregnant?

You need to get to the doctor. We can't diagnose this online...the doctor is where you should be to alleviate any concerns.