Think on milk strike?? My girl also like that . Now about 15 weeks old, for the past 2 weeks no appetite and dont seem hungry at all. Even her previously super-chubby cheeks now like not so fat . She was drinking really little, like 300-400ml a day only. Few days back, I changed bottle & teat, now drinking slightly more. In the midst of changing milk powder too.
Perhaps you can try changing teat too? My boy last time also like that, after change teat, appetite got better. But maybe can observe few days first, sometimes after few days will automatically return to normal. Babies lots of pattern .
Or did baby have jab recently? Or perhaps baby feeling not so well? Hope your baby gets back his appetite soon. I know how worrying it can be. Even now i'm still not "satisfied" with my girl's milk intake but at least better than nothing.