My One And A Half Yr. Old Rott's Face Blew Up On One Side. What Could It Be?

My dog suddenly had extreme swelling in her right jaw. I think it's possible that she was stung or bitten by an insect or scorpion. She instantly became very tired and slept a lot more than normal. She didn't even get up to go potty. I guess she just held it! It has been four days now, and the swelling is slightly going down, but I thought it would go down long before now! What do you think this is, and what can I do to help her feel better?