You haven't said how old your lo is, but if she still has her fontanelle, if it is depressed that's a sign. I don't know if it is the same in children, but in adults, if you pinch the skin and it lacks as much elasticity.
Just from lack of wees, and not keeping water down I would ring the out of hours GP or NHS Direct (who will probably just tell you to do that!)
Let us know how you get on, and hope lo is better soon. xx