Did you ever find out what it was? I'm having the exact same thing and getting no answers!
I would try getting tested or treated for a yeast infection. That might not be the thing, but if it is it's a relatively easy fix, and it'd be good to be able to rule that out. If it's not a yeast infection, at least you've gotten an examination, and hopefully from that they can take a culture and you can get a professional's idea of what might be happening. Good Luck!
Maybe its your normal cyclical discharge which does change from clear to a thicker mucos when you are ovulating...