21 month old has lost weight! Why won't she eat and should I be worried?

Your little one is approaching the 2 year old mark, this is when their eating (or lack of) gets tricky. Having 5 kids and 4 grands, i can tell you it's normal. Make sure she is healthy in all other areas, then just grin and bear it. I used to feed my kids on the run, they would not sit in their chairs at all. They would wander around the kitchen while the rest of us ate and i would feed them that way. A bite of something here and there is better than nothing. Vitamins are a good idea and make sure she gets enough hydration. Avoid junk and sweets, these will not only ruin her teeth and keep her up, but junk will be all she wants to eat.
It will last for a while, but i don't think you should worry much. Make sure she gets a well baby check, mention it to her doctor and carry on. Welcome to motherhood. o.O

thanks. Everyone has been assuring me it's normal, but wanted to hear it from someone who wasn't a relative or friend. I have a 10 month old who will eat anything, so I'm not looking forward to going through this again soon ha!

Another name for this challenging time in all parents lives is known as "The Terrible Two's"
My grandson just turned 2 last week and he doesn't eat enough to keep a bird alive, but this is all part of it.

I'm glad i could ease your mind a little. Advice from friends and fam is always helpful but an outsider's views and advice is also a good thing. It just echo's what you may have already been told.