4 months old drinking lesser than before

Hi mummies, my baby just turned 4 months old and is mostly formula fed. Her usual feed per day is approx. 600+ ml (100-120ml x 6 feeds) prior which i noticed is already lesser than advised. Currently, she is drinking even lesser and cannot even finish a 120ml bottle in a single feed (can only drink 50-60ml each time). Feeds these few days only amounted to 600ml or lesser. Wondering if anyone has this issue too?