I would talk to your Dr about supplements at this point. The main thing is liquids if she is refusing and she cannot boycott liquids, if she is getting dehyrdrated she will end up in the hospital.
Kids go through picky stages, you have to offer it and then if she plays with it do what you are doing. Take her down, she won't not eat because of this, the two are seperate issues. It is only a battle if you make it one. She has a choice to eat or not, period. If you are inconsistent on the throwing food thing she will be confused on when it is okay to listen to you. If she throws it, take her down from the table. If she sees you stressed about her eating it will cause more problems. Be matter of fact about it, if she doesn't want to eat say "well it is here if you get hungry" and put it up. She needs to see you relaxed about eating and meals.
When she is refusing anything nutritional for days on end you need to talk to your Dr. If is just a few days don't worry too much just make sure she drinks.
When my kids went on mini food strikes, I would give them pediasure mixed with regular whole milk, since she is allergic to milk you can try soy. They have yummy flavors and I knew at least they were getting the calories and vitamins they needed. Or go look at a health food store for maybe the equivilant of the pediasure without dairy.
Another thing is you can try too is smoothies without dairy in them. Just add apple juice (Or better the V8 juices with veggies in the, cut up bananas, carrots, ripe peaches or whatever her favorite fruit is and blend up with crushed ice. Add soy milk if need be for more texture.
My daughter was only 16 lbs on her first birthday and has continued to be petite. I used to worry a lot and the Dr reassured me about it. Is your Dr the one concerned? If so you need to talk to them about what a good plan is and your concerns always.
Offer higher calorie and nutritious stuff however don't let her play with her food either. She very well could be teething which causing big changes in what they want to eat and appetite. Try seeing if she is cutting molars. I know during that time my kids just weren't interested in food.
Make sure she is hydrated always though. She should have five to six pee pee diapers a day and there shouldn't be color to her urine (unless she has taken a vitamin).
Good luck, I will pray for you it is a phase.