Oscar had this problem several times, which was originally thought by the vet (and us) to be related to a bladder infection... I posted a thread on it somewhere here... BUT in the end it turns out it wasn't?? Anyway then the vet said maybe it was a structural/growth thing, so hopefully he'd outgrow it-- there are tests they can do to scan for structural urinary issues, but we're currently waiting to see if it's just an "immature" bladder thing... and fortunately he hasn't had this issue in about a month now!! So I'm hopeful that it is just that Oscar is growing into his bladder, or maturing, or something, rather than an infection/structural problem causing the incontinence. I do agree that every time it happened, he had drank lots of water and/or was really playing hard before he napped, rather than on an empty bladder, so there was definitely a "sleeping on a full bladder" component (as distinguished from incontinence situations where urine is just continually leaked from the bladder).
I know girl dogs are a little different, however, since I *think* they are more prone to incontinence-- but maybe that is only after spaying or something (?). Anyway I am hopeful your girl is just growing into her urinary system like Oscar, and will achieve more control the older she gets!
