Swelling, Odd Behavior Problem

[rendons]

Two days ago, my dog, a black lab/springer spaniel mix, 12 years old, had swelling in her face and neck. It didn't seem to be bothering her at all until later that night, when she was pacing and breathing heavily and drinking alot of water. The next day (yesterday), the swelling in her face was gone but her neck was still a little bit swollen. Her behavior was still very strange. She was acting very skittish, anxious, and scared, running and hiding in corners, pacing, breathing heavily, shaking her head alot and drinking alot of water. We took her to the Veterinarian and they said that she was probably experiencing an allergic reaction to something and that it would be over in a day or two, and we could give her a low dose of Benedril to help her. Today, her behavior is still the same, but I just now noticed that both of her ears are very swollen. It must have just happened in the past couple of hours because I hadn't noticed it before.
Should we watch it for another day to see if the swelling goes down like it did before? Is this something more serious than an allergic reaction? Thanks for your help.