18m eating issues

My 18m (16.5adjusted) preemie used to be the best eater until two months ago. She would eat literally anything we gave her.. veggies, fruits, meat, anything! Now at meal times she will look at her plate, and cry and sign "more". She's not a big talker so we obviously don't know what she wants when signing that. Anyway, we've tried letting her cry awhile, and then will give her one of the few things she'll actually eat (yogurt, fruit or bread/crackers). I'm a little bewildered what's going on... it's not like she dislikes what we are serving her (she won't put anything in her mouth, so she isn't even trying it).. she's just refusing to eat anything. This has been going on for two months. I swore I'd never be the mom who makes separate meals (or in her case "snack" meal), but if we don't give in she just cries to the point of gagging and she won't eat anything. Is this a really weird picky phase? Or should we be worried? We plan on addressing it at her 18m check up.