SHe could have a sensitivity to the cold. Then, when she's scratching, it just makes it worse. Or, like others have said- she has developed an allergy to an outdoor allergen. Right now, most pollens are not active. Ragweed season is over, but there are airborne molds. A possibility. The allergist can skin test her for all those things. And, even if she has been eating the same foods, she could have developed an allergy to them. However, most food allergies happen shortly after eating them. Typically, not hours later. Not to say it can't happen, just not as likely. What's better to give her than Benadryl is Children's Zyrtec, CLaritin or Allegra. Allegra has the least amount of side-effects.
PS- I have worked for an allergist for 5 years.
*Most people believe that the first time a food is introduced, it will cause a reaction. It's after the initial introduction that the allergy is produced. So, the body is building up anitibodies to the food the more you eat it.
Good luck!