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Hello. I'm very sorry to hear about your dog.
It sounds like IMT (immune mediated thrombocytopenia) to me. This is an auto-immune disease where the body attacks it's own platelets. Platelets are needed to clot blood. IMT dogs end up having bleeding issues. They can also potentially have blood clots earlier in the disease. Either way, it could be a bleed under the skin causing the bruising and also around the spinal cord causing the paralysis. These dogs can also have brain bleeds or bleeding in the lungs that can lead to rapid and unavoidable death. I think that is probably what happened given the lack of previous signs and the rapid onset and fatal consequence.
I'm sorry again for your loss. Let me know if you have any other questions.