2 month old hungry but fussing while drinking

Ladies, my 2 month old has been drinking lesser milk than usual for the last few days. She drinks the first 1oz really quickly and then fusses and pushes the bottle away. She refuses to drink even after burping her and giving her a short break. She cries and makes gagging sounds sometimes. She does seem to drink after a longer break but only when she's sleepy and we slip the bottle in while her eyes are closed! She wasn't doing this before and drinking 3oz in one shot! The only change we've made recently is slowly giving her exclusively formula instead of bottle feeding breast milk + formula. Has anyone experienced this? Help please!