It either had rot or is still actively rotting from the inside out. My guess is too rich of soil and too much water. It happens....
Cactus exposed to cold would more likely be marked on the top over a fairly wide field, not dots on the lower side.
The good news is the rest of it looks pretty good, not awesome, but nice and green. Look at the very top/apex and see if there are any new spines (this where I look for new growth, it's easiest to spot here). New spines = good news. No new spines still not cause for concern at this point.
If this were my plant, I'd leave it until the soil has dried out completely. By that time it will have either rotted in two or started growing. (Assuming the days are getting longer where you are.)
Long term, I would pot it in mineral-based soil and water when it needs it.