I can only suggest few things:
1. Food allergy is a specific disorder, it goes with face flushing, lips swelling, skin itching and...diarrhea. IgE antibodies and eosoinophils (a type of leukocytes are found in the blood). If this is not the case, she doesn't have allergy. Common food allergies in adults are to fish, shellfish, tree nuts and peanuts and to some antibiotics.
2. Ischemic colitis causes pain in left lower abdomen, and it may cause intestinal bleeding.