Actually from what you described, it sounds more like the mucus plug.
Was it like... stringy?
That happened to me too, I was sexual the night prior and then there was that mucus like substance.
It was my mucus plug that I was losing. Lol. So... Go to your doctors and if they say that you are dilated at all, that could be why.
Sex brings on contractions and the like, and irritating the insides like that of course can cause the cervix to want to dilate. Lol. Just a little experience on my behalf. >>;
i'm 39 weeks along so far, 8 days left til my due date.. Getting close. Scary. ;-;