In my experience, things that contribute to my children wetting the bed are primarily one of three things:
- Drinking more before/during bedtime
- Sleeping more soundly
- Barriers to getting up to go in the middle of the night
(Plus of course forgetting to go right before bed.)
How could cold affect these?
- When it's cold outside, it's also dry. That means the child may be thirstier, and thus drink more before/during bedtime.
- Some children may sleep more soundly in the cold, though it's more common that sleep is sounder in warmth. But, your child may have dressed more warmly and/or put on more covers than usual, and thus ended up sleeping more soundly.
- Having all of those covers on would be a substantial barrier to going to the bathroom in the middle of the night. It also would be cold getting out of them - another barrier.
All that said, given the n=1 nature of this occurrence, I wouldn't read too much into it. Yes, it's possible it's related to the cold. It's also possible she just had an accident. That's entirely normal and to be expected until 6 or so, from what I recall (and some kids, even older than that). I wouldn't see it as regression at all.