According to the Mayo Clinic, breastfeeding mothers should avoid or limit alcohol, caffeine and fish in their diets. Alcohol in breast milk is not safe at any level for babies. Caffeine should be limited to three cups (24 ounces) per day, as it can agitate your baby or interfere with their sleep. Most fish and seafood contains mercury, which can negatively affect the baby's nervous system development.
The Mayo Clinic explains that there are other foods or drinks that, although not necessary to avoid, may cause a baby to fuss or have an allergic reaction. If the baby seems to be fussier than usual after nursing, it could be caused by something specific in your diet. Foods that commonly cause changes in behavior or allergic reactions are animal products, such as cow's milk and eggs, or allergenic foods, such as peanuts, tree nuts, wheat, soy and fish. If the baby starts to fuss or develops a rash, diarrhea or congestion shortly after nursing, these symptoms could mean a food allergy and the baby's doctor should be consulted.
The Mayo Clinic suggests keeping a food diary with lists of everything eaten and drunk and the ways the baby reacts to these foods. If a certain food or ingredient in the diet seems to be negatively affecting the baby, try eliminating that element from the diet for a week to see if the baby's behavior adjusts.