I think it might be part of the mucus plug as well. With my last pregnancy, I thought I had lost the whole plug because I discovered almost exactly what you described--a big glob of what looked to be snot (sorry, if tmi!). But I continued to get smaller pieces of this same stuff coming out over the next couple days.
Your ob should be able to tell you if that's what it is or not if you bring it with you. But as another poster mentioned, I think they would also be able to tell you if you had lost it when they check you for dilation and effacement.