Refusing bottle

My 4 month old has always been formula & bottle fed. The past few weeks she has been struggling to eat a full bottle (was taking 5-6oz every feeding) starts to turn her head away or pushes it out with her tongue after only 3ish oz and refusing to take more (she doesn’t have reflux). Pediatrician isn’t concerned since she is still gaining weight but it’s very hard for me to not be worried and frustrated when this happens. We can’t get into any sort of routine not sure what to do🤷🏻‍♀️We already changed the nipple size and that worked for about a day then right back to the same thing. Wondering if anyone else has experienced something similar. Was it just a phase? Is there something different I can try to do to help?

When my son was a baby he never turned down a bottle lol even when he was teething or had a ear infection so this is so foreign to me ☹️