Not wanting Milk

My sweet little 5 month old has always ate fine but just this past week she's been drinking about half an oz and then gets super upset and wont drink anymore, I'll try again about a half hour later and I can usually get her to drink almost the rest of the bottle but she still will fight it some. We haven't started food yet, we haven't changed formual or bottles, she acts fine and will still have anywhere from 22 to 29 oz a day , plenty of wet and poopy diapers but has been just been fighting me big time out of no where and I have no idea why?? And then randomly she'll drink like 8oz at once... so I have no idea... just wondered if anyone else has experienced this or would know of something that could help?