Well,.....from what I could understand from your post, it would indeed seem that low temps were the cause of the regurges.
Hatchling Milksnakes do well with temps in the low 80's. The worst thing you can do is allow a snake to continually regurge meals, as like Paul mentioned, it severely depletes essencial acids/enzymes, and electrolyles in the stomach needed for digestion. The Nutri Bac will more than likely help the snake recover back to normal, but if this happens again after a week or so of the snake being in the low 80's, it's time to make a direct path to a qualified reptile vet. If the snake has microbial pathogens, then Flagyl(Metronidazole) would likely take care of things administered at a dose of 25-50 mg/per kg of animal weight(repeated in 14 days). I use the higher end dose of 50 mg/kg as this drug is very well tollerated by snakes. I have used Flagyl in the past with VERY successful results, but don't really recommend folks administering it themselves unless they are very advanced herpers, unless doses are rationed out from a vet. It can be kind of tricky for most folks to weigh hatchling snakes without the right equipment to give proper doses.
In any case, just make sure the temps are stable in the low 80's for a week or so until the next attempt at feeding, and then only offer a SMALL meal. If after several very small meals are held down, you could then later work gradually back to normal sized meals.
Hope the snake has a speedy recovery!
~Doug
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"Better to be silent and thought a fool, than to open mouth and remove any doubt!"