Is she growing properly? I would try not to worry if she is growing fine at her last doctor visit and then ask at the next visit. It isn't important how big they are but how they grow on their own chart.
I would try not to resort to cookies and junk food to entice her though. If she is truly hungry, she will eat. It's human nature. If she doesn't then she will stop growing and this your doctor would notice and investigate further.
I would make sure that you aren't giving too much milk. Kids this age love thier milk but then they sometimes fill up on it and don't eat the food. Also, little kids need far less food than we as adults usually think they do. So try to be patient and give her some choices. Good luck.
Specialmom is right, she will eat when she is ready to. Don't worry, just keep offering her food and if she doesn't want it, dont make her eat it. My daughter has been really thin all her life and I got her to gain a little weight by giving her pediasure. That is all she eats in the morning. She is like me and doesn't like to eat any real food until later in the day.
Try giving your child control over what she eats. often times you will find that an intellegent child will struggle more with have every second of every day planned out for her. A strong willed child can have these sort of problems with lots of things... for us it was going to bed and eating that she resisted with such hard headedness. When i started offering my child choices at bedtime like allowing her to pick the story we read and to pick a special animal to sleep with she became much easier to deal with. She actually likes bed time now that it is something she is in charge of. The same idea went for food. We couldnt get her to eat much of anything so I started getting her more involved in the process of cooking. I know that this is a scary thought but try giving her the recipe for playdough (lots of great homemade ones online) to mix up with her hands while you are making super. make sure you let her stay in the kitchen and keep her involved in the process. offer her small decisions like " should mommy cut the carrots this way or this way for your plate" giving your child some control over her day is more likely to help than any kind of cookie...