6 month old Arching Back

After trying to Google research this, I’m at a loss. For the last few weeks, my 6 mo old won’t finish her (7oz) bottle during feeding time. She’ll wake up, take the bottle but it seems like halfway through she’s done and starts arching her back. I burp her and try to continue feeding but continues to arch her back/whining refusing to finish. I have to wait a bit before she seems hungry again to finish it.

I’m a little concerned since her pediatrician said she’s underweight and needs to eat more often. She’s a slim girl who definitely won’t eat a full 6/7oz bottle every 2.5 hrs (doctor recommendation)

Anyone else dealt with this?