4 Month Old Suddenly Refusing Bottle

My 4 month old normally drinks a full 6 oz bottle every three hours. Two days ago, she started turning away from the bottle and barely drinking 3 oz at a time. I haven't changed bottles or formula. She doesn't seem to be hungry, but she's already a petite baby and I worry that she isn't getting enough. Have you guys encountered this? If so, what caused the change and what did you do?