A few weeks ago, my LO was pulling at her ears and not eating well. She wasn't fussy but I knew something was up. When I brought her into the pediatrician, he said her ears were fine but she was starting to get a molar on each side of her mouth!
So if baby all of a sudden is not eating, I would try and gently check for lumps on LO's gums. It could be molars! Good luck and I hope baby starts to do better soon.