I have a very small child who has been below the charts since birth. I have constantly worried if I was feeding her enough.
My advice - which I got from others - is to try and relax. Make your child a plate of food, sit it where they can get to it, and then just go about your business. Don't focus on the feeding. Don't make it a stressful thing.
If the child is always stressed-out about eating, then they can't relax enough to just eat!
Also, you can try making a little snack tray with healthy snacks like small cut-up fruit, small pieces of string cheese, cut olives, soft vegetables, crackers, etc. Leave that tray where the child can access it throughout the day. A lot of toddlers can't really eat a big meal, but would rather just "graze" all day long, small bites.
It's also really normal for kids to stop eating around that age. They just don't need as much food! They aren't growing as quickly as they were as a newborn.
Also, I think you mentioned she's on toddler formula? That's great!! Keep that up. That can fill in any gaps if she does go a few days without much food.
Try to relax. That's the biggest advice I have.