I was going to ditto Lyzelle, IBS just means "you have something wrong but we don't know what it is." Usually it is after they've ruled out some of the serious things. They told me it was IBS after doing screenings for celiac and Chron's and something else, I forget what now. I was on multiple different meds and each one I got really optimistic because I would see people on the internet saying it totally fixed them, blah blah blah, and nothing ever actually helped. Turns out it was gluten all along. To this day I have no idea why no doctor ever told me "Maybe you should just try cutting out gluten."
I would start doing some food trials to rule things out. I was just thinking about the cold urticaria, I know we mentioned it before because I have it and it has been linked to gluten sensitivity... mine has definitely gotten MUCH better since I quit eating gluten. I only get a minor rash every now and again, nothing at all like it used to be (I have a huge tube of benadryl cream I had to keep with me before!)
I'm not saying it's gluten and I'm not saying no gluten will fix everything, but IMO, it's worth trying to see if it helps. They say it takes 30 days (or more...) to really see the effects. I would say that's about how long it took for me. Mine was more like I slowly just started going longer and longer between being sick and then one day I was like "hey... I can't remember the last time I got sick!" So unfortunately it will take some sorta long haul testing to find out if that's it for sure. You can try going to the doctor for a celiac panel but there are false negatives AND false positives pretty often with that, and like I said, I wish I had just TRIED it and I would have found out that was it. I wish I had tried it before spending all that money on all those tests... =P
Dairy is another common one that I would try ruling out. The egg you said she outgrew but it might be worth trying without that again?
ETA: If you wanted to try going gluten free, honestly the easiest way is to avoid most packaged foods. Lots of just fresh (or frozen) foods and fresh ingredients, meat, veggies, fruits. With meat you just have to be careful because sometimes they use food coloring that has gluten in it, most often with ham. You could just try a 30 day "no packaged food" challenge type of thing and see how it does. For me I actually ditched a lot of packaged food first, so when I realized it was gluten making me sick, it was SO much easier to just be like "oh, okay." Because a lot of that stuff I didn't eat anymore anyway. I had bought a big bag of high quality stone-ground whole wheat flour that I realized I couldn't use LOL. But it wasn't too bad. It will be hard if she's a picky eater to convince her to give up some things like mac n cheese or whatever. They do make gluten free versions of those but they are expensive and, well, they aren't going to taste the same. I would make things like that more of a treat. She is old enough she might be okay to get on board if you explain about how it's to try and figure out what is making her sick.
Honestly, when you talk about how she is afraid to eat and then go somewhere, that is what a LOT of my life was like starting around 13 years old... if I had to go run errands I couldn't eat because I didn't know if I would end up sick. I couldn't go out and eat a lot of the time because it was embarrassing to be sick in a restaurant bathroom. I would vanish from the table for 30+ minutes at a time and that was routine... it is such a change now, it's so wonderful, it really is. I hope you can find out what's causing it before she has to live too long like that. It's not fun at all and I can't imagine having to deal with that being as young as she is, she's even younger than me when it started. =/