Can this also mean ‘she was hotter yesterday’?
Yes, that is perfectly correct and is already one of the alternative translations.
"Regrets, I've had a few..."
Are there different words for 'warm' and 'hot' in Welsh?
Generally, although it can vary:
It was warmer yesterday was not accepted, yet this would be the more normal construction in English. Could this be added please?
In Welsh & this course there are different words for warm and hot.