I really hope it's not cancer. Eloise had similar symptoms. The swelling started in her neck and over a couple of weeks progressed to all the the lymph nodes. The vet said it was cancer. We tried steroids before it got too terrible, but that didn't help. I ended up having her euthanized when she stopped enjoying her Yoggie treats and was sleeping more than playing. She was a little over 2 years old.
This is what she looked like. Notice the neck and shoulder swelling:

I really hope your girl responds to medication and it's nothing serious. Keeping all human and rattie toes and fingers crossed.