I'm so sorry, I don't know what could've happened but I understand how you're feeling. I lost my mare unexpectedly to a presumed guttural pouch infection and after we spent hours with her on an IV (after she was found laying out in the pasture), and a constant nose bleed, she just started swaying and toppled over before dying a couple minutes later. If your experience was anything like mine, it's truly a traumatizing way to lose a horse and I still haven't come to terms with it... so I hope knowing that you gave her a great life comforts you even slightly.
ETA: I also want to add that the day before she died, we were just putting her back into a light workload and she was loving it. She was forward and it felt like she was enjoying herself. The only things that were off was that she was slightly girthy and that she didn't rub her head against my curry mitt like she always did, but we figured the girthiness was because she had a week off.
ETA 2: And during her last hours with me, she also had the completely blank look on her face. Her eyes were always so full of life, but it was almost like I didn't exist in the way that she didn't move her head or look around... only if the blood ran into her mouth before I could wipe it or if she was sneezing.