If you've got this far without a reaction to a food then it could be something else is going on but assuming the reaction to food for a moment.
Do you have any history of allergies in your family as that would be the first place to look? If nothing springs to mind then the most common problem is cow's milk proteins. This is an extract from kellymom.com.
Breastfed babies who are sensitive to dairy in mom's diet are sensitive to specific cow's milk antibodies, in the form of proteins (not lactose), which pass into the mother's milk. Cow's milk (either in the mother's diet or engineered into formula) is a common source of food sensitivity in babies. Cow's milk sensitivity or allergy can cause colic-like symptoms, eczema, wheezing, vomiting, diarrhea (including bloody diarrhea), constipation, hives, and/or a stuffy, itchy nose.
If your baby is sensitive to dairy in your diet, it will not help to switch to lactose-free dairy products. The problem is the cow’s milk proteins, not the lactose. Cooking dairy products may reduce but will not eliminate the allergens.
A significant percentage of babies with cow's milk protein allergy will also react to soy. Most dairy-allergic babies will also react to goat's milk or sheep’s milk. Some will also react to beef.
If you think that your baby may be sensitive to dairy products in your diet, remember that it can take 10 days to 3 weeks to eliminate cow's milk protein from your system—allow a full 2-3 weeks of dairy elimination before evaluating the results.
This is also from kellymom:
Most infant fussiness is normal for a young baby, and is not related to foods in mom's diet. If your baby is sensitive to something you are eating, you will most likely notice other symptoms in addition to fussiness, such as excessive spitting up or vomiting, colic, rash or persistent congestion. Fussiness that is not accompanied by other symptoms and calms with more frequent nursing is probably not food-related.
Other common culprits are eggs, soya, peanuts. The allergy board will have some people with experience who can help you.
Could she have changed the way she is feeding to take in more air? If she is teething she could be swallowing excess saliva. Could she have been taking on more foremilk perhaps because you've started to switch sides?
I used to find that tummy massage helped - gently making an upside down U shape was something from a baby massage class. Or making 'bicycle legs' or distraction with something else. I hope you can find an answer - good luck.
