Welcome to JustAnswer, this is Dr. Christie.
Can you take the temperature of your cow (rectally) with a thermometer?
Also is she eating and drinking at all?
Any drooling? Does her breath smell bad at all?
I am not sure where you are located, but is it possible she could have been stung by bees?
Typically lumpy jaw develops over sometime rather than appearing overnight, though I guess she could have had some residual swelling previously? It is possible she could have been stung by bees, has an abscessed tooth or even an abscess in her mouth that is causing the swelling.
If it truly is lumpy jaw, your vet will give her iodine intraveneously to help treat it. If it is an abscess, first you have to determine what is causing the abscess - foreign body, cut, abscessed tooth, etc. Typically lancing the abscess or getting rid of the infected tooth is recommended and then starting her on penicillin and an antiinflammatory such as Banamine.