It's definitely a good idea to see an allergist. Much of the appt will probably seem very much like a visit to the ped. You will probably be asked to give your family's health history several times, they'll weigh and measure your daughter, take vitals, check heart and lungs as well as ears, mouth and nose. Then, I assume they'll be testing. SPT is the most common initial test. It's a skin test. Panels containing potential allergens are placed on her back, each one scratching the skin just a little (It has improved considerably; when my dh was tested 30 years ago, they used little needles to prick the skin). My dd, our oldest, had some testing done in September (she's 9) and she said it kind of tickled, didn't really hurt at all. The allergist then measures the weals on the skin, comparing reactions to a histamine control.
Please keep in mind that all traditional allergy tests can only measure IgE responses. Your daughter's reactions appear to be gastro related; gastro reactions are frequently caused by non-IgE-mediated allergies and won't show up on a test. If your allergist is any good, he/she will acknowledge that the tests are limited and that "reaction trumps test". Too many allergists and other medical professionals will write off non-IgE-mediated allegies as well as other false negatives as "sensitivities" or the result of a mother's over-active imagination. If you get discouraged by the test results or by the dr's lack of concern, just remember that the top allergists in the country would take your dd's symptoms as being an indication that something is going on and would tell you to trust your instincts. If you see an improvement when you remove certain foods, trust that.
A good allergist will also give you a list of foods and ingredients to avoid, as well as substitutes for those foods when applicable. If a test result is high, you should also leave with a script for an epi-pen, jr. Reactions can change abruptly and you'll need to be ready. A daily antihistamine is not needed unless you see a constant problem like scratching or wheezing or congestion--and then, those might disappear when you eliminate allergens. We have no need for an antihistamine unless ds2's having a reaction.