Hey L.,
To some degree, I can understand what it's like to agonize over your child's weight. My oldest son is in the 95th percentile for weight. We eat all whole foods, fruits and veggies and I don't allow pop or junk food in my home. He is outside most of the day playing. However, it's just how he's built. Our prior doctor never failed to make me feel awful, though. I left his 2 year checkup in tears (he's 3 now, and I've switched docs). Anyways, don't let doctors or weight charts get to you. You sound like a great mom, and just the fact that your interested in keeping your daughter healthy shows what a good mom you are.
I have a 10 month old who is quite a bit thinner than my oldest. He's in the 48th percentile. He picks at bottles during the day and until recently, he wouldn't actually drink all of his bottle. I think he may have been getting a few teeth in at once and wasn't very hungry. I've actually bumped up his table food now. He gets baby cereal in the AM and again in the PM before bed. He likes small peice of cheese cut up or cottage cheese and fruits or steamed veggies cut up as well. I also give him whole wheat bread to chew on. I think his interest has really waned in baby food and he wants the real deal now. Your daughter may be tired of formula and wanting to try some more adult fare. I try to include my youngest in whatever we're eating for dinner (pasta, veggies, etc).
Other than that, I wouldn't worry too much (easier said than done, I know). As long as she's not hungry and you're offering it to her, she should be just fine. I actually hate the height and weight charts for babies and children. Why worry mothers with one more detail or find another way to label a child? Good luck!