Awww... my heart goes out to you and to your poor sweetie. You must be so frustrated - it sounds like you have done wonderful job with her diet and getting her to eat such a variety of fruits and veggies and other things, and not giving her a lot of sugar and juice, you must be going nuts. Don't hate it when you stare at your doctor who otherwise seems so brilliant and knowledgeable after asking a nutrition-related question and they just go blank, or say something totally counter-intuitive???
I can really relate because my son had food allergies as a baby (thank goodness he has grown out of them now) that made our food selections limited and it was hard! I was also breastfeeding him too at that age.
A few suggestions: I would see if she will take more water, firstly. My son would down bottles of it, that always seemed to help his system out. I also wasn't producing much breastmilk by that time, I don't know about you, but it was more of a comfort thing for him. Also, fruit purees always are a favorite even at 2 1/2, and I find that they can get things going down there. In addition to, not as a substitute for whole fresh fruit, we do a lot of berry mixed with pear or mango (which is a great laxative), basically baby food but for dessert or at breakfast. It's a great way for me to mix in extra veggies to his diet because unlike your girl, my boy is not crazy for many veggies as is. Nothing wrong with the store bought purees and there's lots of options of organic, squeeze bags, cups, etc. I love Ellas's Organic and especially the sweet potato, pumpkin and apple one. My son loves the little squeeze bag ones and they make great on-the-go snacks.
Pumpkin and prune purees are the two most laxative ingredients I know of. I still keep baby food prunes around for once in a while if needed. If she doesn't like the taste, mix with apple or pear. Pumpkin is a delicious food that can be mixed into desserts as well as soups, certain pasta dishes, and they may even like it by the spoonful. Oh, and one more thing...I have stopped giving my son deli meats because I've noticed it can make his digestion weird, I'm not completely sure that is it but it's happened a few times and so I'm wary of all the preservatives and additives in them. Best of luck and hope she gets all better soon!
post note: sorry I should have read the So What Happened first! Anyway wouldn't hurt to up the fluids if she will take them. Also I don't know if she can tolerate soy but my son started off on soy milk because of allergies and we never switched over. We used to give him the sweetened one and mix with unsweetened, but now he likes it plain organic unsweetened. Once in while will add some honey as a treat. If he's being fussy about not wanting water or begging for juice, I like giving him soy milk knowing it is full of calcium, etc. Also forgot to mention beans... lots of beans!