What Causes Brownish Vaginal Discharge During Pregnancy?

Hi I'm 19 years old and this is my first pregnancy so I'm sorry if this is a stupid question. I'm 39 weeks tomorrow and I had my cervix checked this morning around 10 am. After I got home I ate and took a nap. When I woke up just now around 345 my underwear felt wet. I went to the bathroom and my underwear are completely covered with a brown slimy discharge. I've had my cervix checked before and never had any discharge afterwards. Is this normal or should I be worried? I called my dr and left a message but they haven't called me back yet. I'm not sure what I need to do