Mucus Plug?

I think I've lost my mucus plug and I am currently 36 and half weeks pregnant. I'm pretty sure I've lost smaller white chunks but a huge brown, booger like substance was in my toilet. I didn't think anything of it until I was washing my hands and it hit me. I've been having back pressure, some menstrual like cramping, and I believe some very weak contractions. I have an appointment with my OB on the 28th, but maybe I should call and see if I can get in earlier? Any thoughts or advice?