What does a food allergy look like?

Hi everyone. My dd is 14 months old and has mspi and breast feeds. We have been milk and soy free since one month. For a few months we've been battling diaper rash. Doesn't look like a normal rash to me, more like hives. Her doc said it was not yeast infection. So recently she was eating hummus (which I eat like everyday) and everywhere the hummus touched her face and hands turned bright red and almost hive like, Also she broke out in what looked like a huge hive on her butt. So I cut out hummus from my diet as well hers and the diaper area cleared right up. A few days ago I gave her two blackberries. Within an hour (because that's when I gave her her bath) i noticed her butt had broken out again. Can a food allergy manifest in just one area? There were no other hives on her body. I'm just wondering how you knew your children were allergic or if its an acidity thing or an intolerance thing?