Hello! Can you post her feeding schedule/how much she eats per "meal"? I work with kids who are underweight/have feeding issues, and typically the doctors become concerns when you LO drops percentile wise. If she was born in the 5th or 10th percentile and stays at that percentile, then that is typically OK, she just may be a smaller child. But if she is progressively dropping percentile wise, that is when concerns sometimes arise.
If you are concerned, you could consult wiht a pediatric nutritionist. They can give you some tips to "fatten up" the foods that you are already giving her without adding alot amount wise. They can also recommend foods to try that will give you more "bang for your buck" as far as calories. There are also supplement such as duocal (a powder) that you can add to food to give more calories...