Baby Drinking Less?

Hi Parents! I'm a FTM and lately my baby girl, whose almost 7 months, have been drinking less. She usually leaves about 1oz of milk left. There are days she'll drink all and days she won't. I feel so bad since I don't want it to go to waste. Sometimes when she drinks she makes grunting sounds, too. She has regular wet and poop diapers. Doctor doesn't seem concerned. Her check up was good.

Anyone else experienced this? Could it be because she's teething? Thank you!