My daughter was miserable with milk allergy until we switched her to Neocate. Two rounds of skin prick allergy testing (very quick & easy test) showed she is allergic to many foods and environmentals. Doctors said that the tests are not 100% accurate though (you use the results as more of a guide) and the only way to know for sure is to do controlled food trials.
That's what we're doing now. For us the process is very frustrating because it hasn't gone so well. She's fine with green beans, carrots, rice cereal, apples, Happy Baby apple puffs, and avocado. Sweet potatoes seemed ok at first but then she started reacting after a couple days. Pork and chicken did not go well.
My daughter's reactions seem to be mostly non-IGE allergies (more intolerances, I guess) with delayed, GI-type symptoms.
For example, on Saturday we gave her chicken for the first time at lunch. She ate it fine. That afternoon, she spit up twice, but it was very small amounts and just a clear liquid - like saliva. She rarely spits up any more so this is unusual but not unheard of. We think she might have a tooth coming it so we hoped it was just the result of producing/swallowing more saliva. She slept fine Saturday night.
Sunday morning she projectile spit up - again clear, and a very small amount. But I got a knot in my stomach because she never projectiles any more...she's maybe done it twice since we took her off milk 3 months ago. We gave her chicken again at lunch. That night, her sleep was so disturbed she was fitful every hour. Crying in her sleep, thrashing around...just like the "good old days." Sigh.
We didn't give her chicken yesterday and she was back to sleeping normally last night. I still have so much to learn about food allergies/intolerances, but a good resource is the Kids with Food Allergies website/support forums. Talking to moms on there, I've learned that delayed reactions are not uncommon, and that with some kids, like mine, the only symptom may be the night waking and pain.
I still don't really understand why the food wouldn't start bothering her until after two exposures and a day later. And I don't understand why she would seem fine all day but it's like a light switch kicks on at night. But it's pretty clear when the food doesn't agree with her.
If you have concerns I would do the testing because it can give you some insight. I would definitely try to find a good allergist (I've learned they can be hard to come by - I still don't have a good one!) that can help you come up with a plan, b/c this stuff is complicated!