I believe I have PID

I am 18 and I have been having dark brown thin discharge for a couple of months now. It stops for about a week or two but then it comes back. At first I thought it was a side effect of my Nexplanon and my doctor told me it could just be old period blood but this has been in place of my period for months now. Sex isn’t painful throughout only in the beginning and afterwards but I believe it’s because of the lack of moisture my vagina produces as a side effect of the Nexplanon. I always have abnormal pain randomly and I will be going to my doctor tomorrow but how can I ask to check for PID if she believes it’s nothing wrong?