I am so very sorry you lost your Macy, and so suddenly too. It's a horrible shock. Please accept my condolences.
A little rice wasn't going to hurt her, I don't think that's the problem at all. I'm sure she didn't just get rice to eat all the time!
Dogs are naturally pretty omniverous. Yes, some human foods are bad news, especially if that's their only diet, -some "human" foods are actually toxic, like avocadoes, chocolate and products containing chocolate, and onions, etc but little bits of rice, potato, vegetables, etc won't do them any harm at all.
It sounds very much to me as though Macy suffered seizures at the end. It came on her suddenly, and I don't think you did anything wrong there, especially if she normally had good health, and had never had a seizure before. You were not to know this was going to happen to her.
The cause? I don't know. If it is intensely hot where you live, then yes, it IS possible intense heat, exercising in heat perhaps, and possible dehydration could have contributed....but I think it was probably one of those sad things that just happened, and you had no control over. She could have had a genetic pre-disposition to seizures for all you know.
I am very very sorry.
I'm so sorry, but sadly this happens more often than not. People become so close to their pets, they forget they are not people. Dogs require good diet, not table scraps and food people eat. We all made this mistake. When you think you are doing good for your dog, you find out later you made big mistake. There could be a dozen reasons you dog passed-on, but hard lesson learned. Dogs are smart and my dog Jack had sickness
from about the first day we saved him. He would have good days and bad, but learning after he died he had liver problem that could not been fixed or operated to repair did not relieve the hurt and guilt. You will feel all these
and that is normal. I know you cared deeply for your Macy and I'm sure she did you. I'm so sorry you lost her at young age of 7. Just remember
to always see a Vet in the future and feed your dog proper diet. I know your love for Macy was not a problem. Hopefully you will save a dog, because so many need a good home. I would wait a while and go through the sadness of your loss. Macy and you deserve it. No dog can replace another. I to this day miss my Jack. He was the best dog. I know you feel the same for Macy. Read Rainbow Bridge and stick around for more support, because there are a bunch of kind and smart people on here who's lost a dog and know your pain. Take Care, Mark