question for you. did you add a few items at a time to the diet? if so back off all of them. you need to add only one item to the diet per 2 weeks that way you will know which item is causing the allergy. also some formulas can cause food allergies as well so that may be whats causing it. what type of formula are you using?
I recommend the following:
1) since your adding formula for the first time. Just stay with that for two weeks and nothing else, unless your adding in breatmilk. But no solids of any kind yet. This will tell you if the baby is having a reaction to the formula.
2) Next. Solids. Start with adding rice cereal only. No other solids yet. Do this for another two weeks. Again you should know at the end of two weeks if the baby is having a reaction to it or not.
3) Add the easiest digestable baby foods next. But only one at a time. A great one to start with is apples or applesauce. Only add it to one feeding. So at this point the baby is getting formula/breastmilk for breakfast. Lunch is rice cereal and milk. Dinner last meal of the day is milk. cereal mixed with apples for taste. Again do this for 2 weeks.
4) Repeat the above step for each new food you introduce so you know which food is causing allergies.
5) Once you have done this with baby for all foods you know you will feed then you should be able to add more than one food a day. Example : breakfast: milk, rice cereal w/apples for taste. Lunch milk, spoonful of veggies and spoonful of fruit.
6) It's a slow process but I have 2 children and this is how we done them trust me it's well worth it to take your time and not upset their stomach.