Dehydration is measured by input and output. If both of these are down, then you need to rehydrate. Just drink water, but not a lot at once, Drink smaller amounts over longer periods of time. If you are urinating normally and your urine is not dark and cloudy, you are not dehydrated.
pinch the skin on the back of your hand together, if it sticks together then you're dehydrated...
That's what I was going to say :(
If it doesn't stick up, you're fine. You just need some rest.