3 months old stomach pain

Hi mummies, my lo has been rejecting her milk. Till now her intake is only 1/3 of her usuals. She has been crying after taking a few mouth. She also has been sleeping the whole day, super lethargic and keep crying every now and then. Her usual poo is yellow and paste like texture. Her poo just now was dark brown and mucus like. Pls advice I should still monitor or to bring her to the doc asap.