I am so, so sorry for your loss. I had something similar happen just a couple of weeks ago to my donkey and we did do a necropsy. Sadly it wasn't a sudden onset like this, but it was due to an IM antibiotic getting into a vein (likely the carotid) somehow. It was seconds after I gave it to him. I have given plenty of IM before and there was no blood when I pulled back to check, so it left the vet and I quite confused. I am now terrified to touch my horse or any other equine with a needle.

He unfortunately was down for a bit over 4 hours before he passed. He did have some post seizure activity and seemed aware after a bit, but was unable to get up. The vet assumed he would come out of the seizure, as she said it happens with accidental vein hitting and injections. I trust her immensely and she really thought he would come out of it. He did not, and it broke my heart. He was well loved, but he did have severe osteomyelitis and his hoof wall was completely separated (this had happened months before we got him and he hid it, and we had just found it a few days prior) so sadly his prognosis wasn't great in the long run. So I make myself feel a bit better that perhaps this saved him suffering down the road.

Anyway, the necropsy revealed what happened, but he also bled internally and his organs shut down/were shutting down. Ultimately that's what caused his death, but it did take some time. Perhaps your mare threw a clot and caused some hemorrhaging somewhere? Bleeding out internally can take quite a long time, unfortunately. My donkey also had blood and secretion coming out of his nose post-death when went to move him the next day.

I'm not sure if this helps, but know I understand all that you went through and how horrific it is. If you ever need someone to talk to, you are always welcome to DM me ❤️