by the sounds of it i would quite happily bet on overwatering, the cold shouldn't be the problem just stop them growing. I am using hempy buckets with pure perlite but started my seeds in jiffy pots, they started out fine but i put them in the perlite still in the jiffy pots and watered loads, for the next weeks my seedlings stopped growing and started turning yellow and went really droopy, so i pulled one out and noticed that the roots hadn't made it preperly into the perlite, i washed all of the jiffy cube stuff off and planted the bare roots into the perlite and then did it with the other 3 plants (not suggesting this, might damage the roots but it worked for me, mabye just let them dry out). they then got better and are thriving now.
are you watering directly near the plant? if so i would say you have the same problem, the roots can't breathe. let it dry out a bit and then water around the edges of your pot