If you haven't got a heat pad, fill a big pop bottle with hottish [not too hot!] water and wrap it in a towel and put it with the pup so it has something to snuggle against. You will have to get up and check it at intervals though, as it is very cold at night now, and as the water cools it will chill the pup, so will need refilling.
The floppy head could have been because it was weak from lack of food, it's still very young and needs to feed frequently so if the bitch is not feeding over night you will need to keep hand feeding during the night.

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