Sounds like your bodily fluids to me. I get the same thing. Im 38 weeks and find that I get very very "wet" whilst DTD. Its to a point of running down my leg and even my hubby feels it and thinks Ive wet myself lol. I find its been getting worse from 30 weeks on, so most likely just the extra weight and pressure, cause I certainly hadnt orgasmed yet.
hope that helps, if your worried Id go to the doc but, or even ring the midwife or hospital and ask for some advice (not giving your name or anything if youre embarrassed)