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Your doctor is right, it was probably your mucus plug. It's nothing to worry about. It rarely comes out all at once and I've heard of women having it come out multiple times, meaning that their body replaced it after it came out.
It is the plug in your cervix that helped make a little seal. Some women lose them right before labor others over time. I lost a small bit a week before my water broke with my daughter. And with my son I lost some pieces like a couple weeks ahead but I lost most of it the morning I went into labor. It is nothing to worry about and unless your water breaks or you are having contractions there is nothing to worry about.
I lost parts of mine three separate times in two days with my daughter but still didn't go into labor. I had to be induced about two weeks after that happened. With my son I wasn't aware if it came out before I went into labor or if it came out at the hospital. I'm almost 39 weeks with my third and haven't lost anything yet.