Baby used to drink a lot but not any more

Hi all, my baby boy is currently 3 months and two weeks old. When he was in his first two months or so, his feeding (bottle) was pretty consistent and he was able to drink up to 130ml in one go every 3 hours or so. We attributed it to a growth spurt. However, for the past two weeks, his feeding became more erratic. He now never drinks more than 50ml per sitting and would also cry, so we have to feed him more often. Is there a particular reason as to why he is acting this way? We have upsized his teat as well.