Does anyone know what fluid behind the uterus indicates (if anything)?

I went for an ultrasound today because I haven't had a menstrual cycle in FOREVER. I was expecting (and kind of hoping...because then it would be somewhat easy) to find cysts on my ovaries, but no, the only abnormal thing was fluid behind my uterus.

I'm just looking for some answers, so if anyone knows anything, please share!

Otherwise, I'll just have to wait until Monday to talk to my gyne...