12 week old reduced milk intake

Hi, FTM here - my 12 week old is exclusively formula fed and is suddenly not taking much milk the past few days. She’s not finishing her usual feed amount and doesn’t seem hungry, yet appears alert and responsive when awake. She seems to fuss if we try to continue to feed her. Diaper frequency appears the same except she occasionally poops once in 2 days. should I take her to the doc to check it out? Am super stressed :(