My 13 month old son some days eats well, other days I (or daddy or grandma) can only get a couple bites in him. He does about the same for daddy, me, and grandma (who watches him 5 hours a day). What I have found is all you can do is offer healthy food, a variety of food, and find creative ways to serve it. I have found that he REALLY likes certain foods such as peas and crackers. So, when he is really fussing and refusing food, I put some crackers or peas on his tray, and he magically starts eating them. There is other food on his tray and then it's as though the crackers are a "stimulator" and he starts to eat the other food, too. I read in Dr. Sears book that this is an idea that he endorses. So, maybe find a food he really likes and try this idea.
According to Dr. Sears, it doesn't matter what toddlers eat each day, it's what they cumulatively eat during the week. He also says what I said before that as a mom (or care provider), your job is simply to offer healthy food in a creative manner and it's up to her to eat it. It's hard for us not to worry, though. I've tried hard to just relax on the days he refuses to eat and say "there's always tomorrow"! He also suggests allowing toddlers to graze throughout the day by having out a graze tray (ice cube tray with healthy food in it) that she can nibble on throughout the day instead of set meal times.
Unless she really starts to lose weight, I would just say it is her age.