12 week old boxer suddenly lethargic, won't open her mouth, moves away when we try to touch or look at her mouth, jaw, neck or throat... didn't see/hear her injure herself. When she does open mouth to yawn, it seems to tweak her neck somehow.

Small sharp object stuck in her throat or further down? Or injury to neck vertebrae that you didn't witness? VET, ASAP for at least an X-ray.