In addition to a possible pyometra, unspayed females are also prone to developing uterine tumors. If it is purely blood that is coming from her vagina, it's possible that she has a uterine tumor that has ruptured. If this is the case, it can become life threatening very quickly since it can lead to hemorrhaging, which can cause her to bleed out within a short period of time.
You need to get her to the vet ASAP to get a diagnosis on this. Pyometra can be life-threatening as well, so no matter what the diagnosis turns out to be, this is nothing to mess around with. There are no good scenarios for these symptoms. Please let us know what the vet says.
Ghilly
Curious. Any updates on your dog?
I would be concerned and contact the Vet. There us a chance this could be pyometra.