my gut instinct is that there's no allergy / dietary issue going on as she's never constipated, sick or has diarhorea yet she eats something from all food groups (at times). bellababy, did your daughter have allergy symptoms before her diagnosis?
it's interesting as she isn't picky in the usual sense of the word, where she limits the types of things she'll eat; it's more that she maybe hasn't got an appetite or is so ambivalent to food that something has to really interest her for her to eat it.
She wasn't "sick" all the time but did have a low grade temp pretty much all the time, 99 - 100, depending on the day, but still acted just fine. The ped said she just had a higher body temp. She was also off and on constipated, but nothing too telling. So I think that's what took so long for the docs to diagnose. But I described exactly what you are describing your daughter. I kept telling the ped that it was like she didn't have an appetite. She also dropped significantly on the growth charts at around a year to two years and the ped just said she was healthy and not to worry. Well I did worry, and for good reason. If it's worrying you there more than likely is something going on. May be something small but trust your gut. You know her best.