Clear liquid and blood?

I use a cup so I know exactly what my flow is like, the consistency of my blood, the color and even the smell. I know what's normal, I feel really in touch with my body and it feels good.

But this cycle is strange- there is a clear liquid on top of the blood when I pull my cup out. And, it isn't just a little- like half the cup is filled with this clearish liquid and half is blood.

Has this happened to anyone else? Thoughts on what this could be?

I know it's not urine because as anyone who uses a cup knows- you can't pee and pull the cup out at the same time unless you like peeing all over yourself! Lol