Baby taking lesser amount of milk each feed

My Bb is 3 weeks old. She used to take about 80mls FBM and finishes it in 1 feed. Recently she has been 'taking sips' like 20-30mls during feeding time and ends up having multiple feeding, that it can get quite tiring at night plus quite wasteful to throw the milk away. Any idea why there is this regression or sudden drop in intake per feed?