N.S.
It sounds like she's probably not fully able to control her bladder yet. I'm guessing she's sensing she needs to go while sleeping, but not able to get to the potty fast enough.
Maybe put a potty chair in her room where she can get to it quickly, and be patient. It might be too soon for this...but check with your pediatrician about a special alarm that attaches to underwear, to alert heavy sleepers as soon as they begin to go to the bathroom so they can get up before having an accident.