Is she eating enough?

My daughter will be 18 months old on the 20th, and weighs 19-20lbs. She is an itty bitty little thing and her dr is not worried about her size. Well, she wasn't at her last dr's visit 3 months ago. She see's her again on the 23rd. But I am. My mom and dad tell me I was tiny when I was little too. Unfortunetly I have NO idea what the father was like when he was little, he left when I was 5 months pregnant. She is active, learning, happy, talks, runs, plays. But she's so small. She eats like a bird and is picky majority of the time. I feed her the same foods I eat (within reason) as well as healthy toddler snacks. I give her one Pediasure every day (recommended by her dr and a nutritionist) and it does make me feel better. But I still worry. Am I over reacting? Everyone says she's fine, and most of the time I think she's fine too. But I still worry. She can still fit into 12 month size clothes. I have to pin EVERYTHING. I put her in cloth diapers at home (pre-folds with the Gerber rubber pant covers) and the covers are huge on her (also 12 month size). I am a poor single mama and was only able to get her to see a nutritionist because WIC required it due to her size. Anyone else going/have gone through this? Any tips or helpful hints on food/snacks/meals would be greatly appreciated. TIA!