When a child's appetite changes or becomes much less than usual can be quite normal. The main thing is that she has her fluids, water, milk, fruit juices.
As your child develops and grows, you may notice that she may want more to eat and that is quite normal too. When children get "growth spurts", they tend to eat more.
If you daughter is well, has no fever and does the things that she loves to do, I would not worry about her not eating much as long as she is getting her fluids. Do offer different drinks and not just the water. Offer her fruit squashes, milky drinks, and soups too. If she does refuse these drinks, then give her what she wants to drink.
The time to worry when a child loses their appetite, is when they are unwell, have a fever, will not take part in things they love to do. In this situation, you would need to see how she was and if there was not improvement or she was getting worse, then a doctor's appointment would be in order.
As a child grows, his/her taste for foods can also change and they may become more picky with what they choose to eat.
If your child is well, there is no need to see a doctor. It is very normal for children of all ages to go through phases of different eating patterns.
The other thing to watch is if she is being fed with sweets, chocolates, biscuits, cakes or other foods without your knowledge. These will spoil her appetite.
Report her symptoms to her doctor and arrange a checkup. If she becomes lethargic / listless, I urge taking her to E.R.
She's eating, but drinking more water than food. Unless her environment is unusual (e.g a hot desert), while sounds like she is managing o.k., her behaviour's strange enough to have her looked at.
She's not grossly obese / fat?