Hi there... I am feeding my DD as we speak so please excuse any typing errors :)
What I am going to write are only suggestions. Your best bet is to contact your pediatrician and get an appointment.... since your babys' issue could be a number of things.
1. Infant formula is always he same, where your breast milk is constantly changing. Please remember that, like pregnancy, anything thay enters YOUR mouth evers your babys' mouth. Over the counter meds, soda, rich foods... even water. Maybe it could be something in your diet.
2. Slight Acid Reflux? Formula is thicker than Breastmilk... therefore making it easier for baby to keep it down.
3. Perhaps her feeding position? This suggestion isnt meant to tork your melon... but pay attention the next time baby is both bottlefed and breatfed. What are the main differences besides her meal?
4. Your let down. If its too fast, there is alot of gulping... too slow, alot of highly pressured sucking. Maybe do a pump session and see how the plumbing is running right now,
5. Your stress. Babys can pick up on it pretty easily.... are you starting to get stressed when you are breastfeeding?
6. Possible allergy to your breastmilk. I dont know alot about this subject, but I have heard it happens. This is something a DR could better help you with.
Good Luck to you guys. Just try not to panic and maybe set up a dr visit.
Also I would like to add... Breastmilk is awesome for babies... doing breast and formula is a great option for many families... your going great :)