is this normal?

[sandrc]

my 27 month old daughter has been potty trained since april. she's been doing very good at going to the potty and having no accidents. the problem is she is holding her urine. sometimes she will go from 7pm all the way through the night to 6am or later, most times later. i sit her on the potty and she just won't go. i ask her if she needs to and she says no. when she does go, most times it's alot and it's yellow. if she waits a long time it has a strong urine smell. she is nursing still (several times a day) and she drinks water and milk sometimes tea or root beer.
i just don't get it. why won't she go pee? she doesn't act like she's in pain or anything. she doesn't say it hurts, she's not red or irritated looking down there. is this something toddlers do or what? i can't imagine this is healthy for her. i try to get her to drink more water to make her go potty but it doesn't work. HELP!