Is your daughter also fussy with the bottle. Make sure you are using nipples that let the milk out slowly. If she doesn't have to work to get the milk out of the bottle, she may ger frustrated having to work at the breast.
Also, are you eating anything that may be upsetting her stomach, such as tomato sauce or a lot of dairy? My daughter would get really fussy whenever I ate a lot of tomatoes.
Where are you breastfeeding? Is it somewhere with a lot of action? I think my daughter was a little older than 3 months when I had to start feeding her exclusively in a chair in the corner of my bedroom with no one else around. Otherwise she would get too interested in things around us and not eat well, although she was hungry and would be fussy.
Finally, she may be teething. Is she drooling, chewing, have a runny nose, etc.? Try some tylenol before she eats and see if that helps calm her down.