Results are in from Purdue, and although they are waiting for more lab results, it appears that Annie had Hemangiosarcoma.
It's a rapidly growing, highly invasive variety of cancer, almost exclusive to dogs, rarely in cats, and almost never in horses. The tumor often appears on the spleen, right sided heart base, or liver. It causes right sided heart failure. It is most often diagnosed post mortem as it doesn't start causing problems until late in its development and those problems are catastrophic. Aggressive and early treatment can extend life by a few months, but it is a fast moving and %100 fatal cancer.
The above video and the video below are Annie EXACTLY one month before her death.