My baby is 9 weeks old and she's formula fed. Just a couple of days ago, her milk intake greatly reduced to 450-540ml per day instead of the usual 720-810ml. Her poop is normal and She's always sleepy throughout the day and night. Any ideas?

Sounds like a growth spurt to me. There will be a few days that they will be like that but as long as their s*** and pee is ok, after a few days she should resume her active ways and drink back nearer to her usual. Just check that there is no discomfort for her like a bloated/gassy tummy. No need to worry :)