My 13 year old son does this. He goes through periods when it's like he has two hollow legs and he needs to fill them up yesterday. Thank goodness his favorite foods are fruits and veggies! Then, there are days when he eats so little, I'm afraid he'll blow away in the wind. He's been this way since birth.
My pedi always told me that if children are growing and developing in all other areas normally, not stress daily intake but try to look more at a weekly intake. She also said that if I was really, really worried (read: not willing to accept what she said), that children's vitamins and a glass of milk per meal would be sufficient. I was told in no uncertain terms that my son would NOT starve to death; he would eat when he was hungry.
So, even though it drives me bananas--I need three meals a day or I'm a basket case--I've had to accept that my son knows his tummy better than I and to trust it. I just make sure I have healthy foods around for when he starts eating. It's gotten easier with time.
In your case, I would say that if your daughter is acting normally in all other areas, don't stress too much. Focus on making sure everything she does consume is extra healthy while she's like this (milk first, then the rest of the meal). If it continues for more than a few weeks, call the pedi for an appointment to see if all her stats are where they should be.
Good luck.