Pregnant Cat Has Dark Brown Discharge...what is it?

I have a pregnant cat that has been having dark brown (red tinged) discharge since last Wednesday night. She is acting normal and eating, but I am wondering what this discharge is, and I'm slightly worried.

I took her to my vet last Thursday, and the vet did an x-ray and said there is one large kitten inside, and that she should have the kitten soon. The vet did not know what the discharge was, but when I called the vet a day or two later, she said that sometimes there is discharge the week prior to birth.

If anyone has experience with something like this, or knows what it is, I would appreciate some advice.

thanks