I don't want to be an alarmist, but please have someone check out her heart. My twins were great eaters but at 3 1/2 months, one of the boys just stopped eating. He screamed when I put the bottle near him and was getting about 10 ozs of formula a day versus 32 that the other twin was getting. I kept thinking it was a digestive issue. This went on for 2 weeks... kept calling the doctor who told me not to worry. Fast forward to Thanksgiving weekend, his heart was racing and he was breathing heavily.
Turns out he was in heart failure. He had an atrial flutter and apparently, when you have a heart issue like that, sucking a bottle is just too strenuous so they push it away. After a week in ICU and 7 months of meds, he was TOTALLY fine (it was just an arrhythmia but b/c it went on so long undiagnosed, his heart was enlarged to twice its normal size and his lungs had filled with fluid).
So, while my case was an extreme one, I always encourage Moms to have an EKG done to rule out a heart problem. (My twin wasn't born with it, so don't listen to your doctor if he/she says there was nothing wrong at birth.... also, the problem my son had could only be diagnosed early on with an EKG and wouldn't be picked up with just a stethoscope b/c it was an atrial problem, not a ventricle problem - - you hear the ventricles on stethoscope).
Good luck and I hope nothing is really wrong.