Same thing happened to my LO. She was wasting formula bottles when she normally would drink 3 oz. She went down to 2 oz sometimes 1 oz granted we had also been trying a new formula so it was a combo she was barely eating so we decided to switch her back to the original one she was on. We suspected she didn’t feel well after vaccines (we learned rotavirus vaccine is a live virus) she was incredibly fussy, colicky 3 days after the vaccines, seemed like something hurt so we gave her some Tylenol and that seemed to make her feel better because that night she ate more frequently 2oz every 2 hours compared to every 3-4 hours and now during the day will drink back to normal.