4yr old is too worried about what is healthy and what isn't

My daughter is 4 1/2yrs old. She only weighs 32 pounds. She doesn't ever want to eat junk food because it isn't healthy. The other day she ate a donut for breakfast and her daycare teacher told her that was not a healthy breakfast and that she had to eat a full bowl of cereal. She is always telling me that she doesn't want to snack because it isn't healthy. Does she have a problem? I'm worried about her. Last week she wanted oatmeal every day for breakfast, but I could tell she didn't like it. She would be gagging it down, but her daycare teacher said it was healthy so she is making herself eat it. Could she have an eating disorder?