My guesses:

1) Premature egg
2) Some kind of cyst that was ruptured during mating
3) Some kind of tumor that was ruptured during mating

Regardless, I would definitely get her to the emergency vet ASAP! That's not something I would even wait for normal business hours for. I'd be on the phone with the emergency vet down the road even as I was driving there. Scary stuff.