Baby won't finish his bottle

My 9 week old (about 9 lbs) was up to eating 4 ounces every 3 hours but the past two days he refuses to eat! He'll only take about 30 or 40 ml at a time, is hungry an hour later and fussy between. He barely napped at all today and I wasn't able to get anything done because I was constantly feeding him. He's on pumped breastmil fortified with formula powder.

Has anyone else gone through this with their little one? Is it gas?