Fluid in the uterine lining?

Does anyone have information about fluid in the uterine lining? During several failed IUI's and a failed IVF cycle, my RE noted fluid in my uterine lining. He's not sure what it is...blood, cervical mucus, etc. and he says out of thousands of patients throughout his career, he's only seen a handful of women who have this strange fluid in the uterine lining. I tend to have a thin uterine lining in addition to the fluid. Any information would be helpful...