David -- I'm so sorry about your girl.
It's hard to say what happened; something obviously spooked her and caused her to get injured.
Have you been able to identify what she cut herself on? I'd try to see if you can find what caused the injury so you can remove or fix it.
It's certainly possible she had some serious internal injury. Very possible.
For the bleeding, my guess would be that she was fatally injured with the cut. It's possible she held her head/neck in a way that cinched an injured vessel closed -- sort of like bending a water hose to stop the flow -- then when she changed position, she un-bent the "hose" and the blood flowed.
It's also possible she bled internally first, but only had a small cut on her skin. The pressure of the blood pooling under the skin causes the small cut to enlarge, causing the blood to flow externally.
For what it's worth, there's nothing you could have done. And she likely passed pretty quickly from the blood loss. She would have gotten very tired and then just went to sleep.