PLEASE dont worry! I *know* this is easier said than done though. My son, now 6 1/2 yo, is a tiny thing that was not even on the charts. His health has always been good and he has never lost weight (without reason - being sick for a few days can cause a slight weight dip, and that's normal and explained. You only need to worry if and when she is sick like that every month and or seems healthy but loses weight anyway.)
AS LONG AS she is healthy, growing (slowly), eating (albeit very little) alert, active, engaged, learning, all that, SHE IS FINE.
My 1st grader as I mentioned is 6 1/2 yo, and yesterday we were over at a friend's house with her 18 month old daughter. The two of them when squatted next to each other, were THE SAME SIZE! He is taller, and maybe 2 lbs heavier. Sheesh. I can worry and beat myself up for his tendency to eat like a bird and try and force him to like fatty foods, or I can not worry and delight that I can *still* pick him up without breaking my back if he's not cooperating or not worry too much if he climbs onto a 18m old baby ride toy. Clothes fit him longer and it is easy for his friends to hand "down" some outgrown clothes.
Look at the positive side, and know that the charts are just that- charts of averages - not real people. We all know those people whose tots are in the top 110% percentile of their height/weight charts - SOMEbody's got to balance those heavy kids out! Better that your child be small, healthy, active, have a healthy eating habit than destined for childhood obesity or later weight/health problems.
I have also found that STOPPING the snacks or all-day-long grazing has helped my son eat more at mealtimes. After school he is ravenous (no wonder, usually his lunch box comes home still filled?!) and he eats and eats for an hour or two, but finally about 4 -4:30 I have to put my foot down and say NO MORE, WAIT for dinner (usually abt 5-6). Cuz he'll finally get full about 4 30 to 5 and then be too full and only pick at dinner and then at bedtime want dessert. No. Snacks are not a subsitute for meals. You don't get dessert if you haven't eaten a good dinner. Period.(even if I'd love to see you eat more and put on a few more pounds)
He does eat very well, he eats a great variety of foods (just doesnt like raw tomatoes, and that's ok. I dont like olives. To each their own. I've told him and he now says that he will grow up to like tomatoes, just doesnt like them yet.) he just doesnt eat very much of it. I have not allowed him to become super picky and only like one or two things like mac n cheese or chicken nuggets every day. He eats what his dad and I eat. I do not cater to him but nor do I serve truly yucky (to a kid) things, at least not without also having bread or salad or something else I know he will eat. Make sense?
As a baby/toddler, he would NOT eat anything if I put butter on it!! I think it wasn't until 3 or 4 yo that I could finally put a little butter on his toast/pancakes/etc. He did NOT like peanut butter until the end of Kindergarten (I'd make him Nutella sandwiches and gradually mixed more peanut butter in it until he was fine with PB) He has never liked milk, but now he will drink chocolate milk occasionally. I dont push foods on him just because every little kid drinks milk or eats PBJ sandwiches, and in fact, I think the overreliance or over-use of certain foods(things) are what causes allergies. So, I let him take it at his pace, slowly introducing over time.
Well, that was a long spiel from my soapbox! :)