Let me start with a question. Has she always been fussy?
My wife and I have a 5-year-old girl and a 4-month-old boy. Within the first month we noticed our new, breastfed boy was very fussy. While bouncing him on an exercise ball, we discovered he had a lot of gas. My wife cut dairy from her diet, and the fussiness all but disappeared. She drank rice milk, ate goat cheese, and yes, even ate dairy free pizza. It wasn't simple, but it was nowhere near climbing Mount Everest either. Now, he's fine. She's added cheese, yogurt, sour cream, etc back to her diet. This week she drank her last rice milk. He's doing great.... You might do a test. Cut don't eat or drink dairy for a week. (It can take that long for traces to leave your system.) Slowly add back a little dairy at a time. Cheese on a sandwich. Yogurt on your cereal. Watch her reaction. And, good luck....
(BTW, love the name Kate. We planned to name our second child Katie, if he had been a she.)