I posted recently about doing allergy testing w/ my 2 year old so you can refer to that for details. All I can say is it was SUCH not a big deal. When I called and talked w/ the allergist prior to our appointment and expressed our concerns she said they do small children all the time.
I did quite a bit of research about food allergies and I will share what I learned in a nut shell...
There are really two things here Food allergies and food senstivities
Food allergies- these foods triger a histimine reaction in the body and can result in hives, rash, diahreah and in the most extreme anaphylatic (throat swells and closes).
Food Senstivities- These foods will not put you in mortal danger but the irriate your body. In children they can make you crabby, over emotional, and cause a short attention span. They can also cause tummy trouble and diahreah, loose stools.
If you go in for a skin prick/sratch test they will only do a handful of the most highly commong allergic things. After doing an intial interview and exam they will determine which things they should do based on your situation (diet, home, evironment, etc.) So like for us b/c we are in a brand new home and Amy sleeps in a brand new bed w/ brand new bedding they did the initial dust mite test and b/c it came back neg didn't feel like the secondary was warranted. They also only did a handful of pollens b/c we didn't susupect pollen allergies w/ her. For food they did egg, wheat, peanut, almond, milk,soy, gluten . I know they did a couple more things but I can't remember what.
We tested positive for peanuts (which I susupected already.) Peanuts are interesting b/c they won't make you react right off. They are one of those things that has to build up in your system and the more you have it, and the more frequently, the more you react. And then you lose sensitivity to them with time. In fact the Dr. said keep her peanut free for 5-7 years and then she may be able to have it.
Anyways with food sensitivity you have to do the elimination diet as noted above. Keep a journal and write down absolutely everything she eats. Then note loose stools, rash, irradic behavior. Maybe she has canteloupe 3 separate times and climbs the wall after each episode. (according to the allergist a highly allergic food! Who knew?)
You can also get a blood draw and have them look for auto immune resoponse to a large variety of foods. This looks more for food sensitivities than allergies. We have not done this yet but may in the future.
My girlfriend has a son w/ autism (aspegers) and he has food allergies/ sensetivities) to everything under the sun. Getting his diet under control has made a huge difference in his behavior.
I used to think a lot of this was whooey, but dealing with it first hand has helped me see that there really are things maybe we best not eat. The global market has opened up some foods to us for year round consumption and these are things that we normally wouldn't have eaten year round or even at all (think macrobiotic.)
Sorry this is so long, but hope I helped shed some light on something for you. Sounds like she's reacting to something, I'd definately keep persuing it.