What Causes Pelvic Inflammatory Disease?

I got diagnosed with pelvic inflammatory disease. the doctor checked for stds, the test came back normal.. no stds! I was curious if the doctor could have mistaked pregnancy for pid. I've been very nauseas in the morning, somedays I throw up. this has been going on for a week and now I've realized my period is a few days late. can pid cause me to miss my period? or is there a possibility I'm pregnant? I'm 17, I am sexually active, and i have never missed a period. please help!/: