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Hello and thank you for using this site to help you. It sounds like your dog had mild hypothermia and this happens when the body cannot regulate the temperature properly. Young dogs and senior dogs generally can have problems regulating their body temperature so precaution should be taken as to where they are kept as well as the amount of exercise they get so they do not lose heat from the body.
Dogs with a thyroid problem or low blood sugar also cannot regulate their body temperature well.
I am going to direct you to a site that goes over this for better understanding.