Head Tremors in 6 month old Puggle :(

[picklz]

I'm hoping someone has some ideas about what is going on with my fur baby . My 6 month old female puggle has been having head tremors. It started about 1-2 weeks ago. The first time it happened we had just woke up her head started bobbing almost like a shiver but it was only her head. It didn't happen for about 1 more week then it started happening more frequently. She has also had a higher body temperature 102.6 F and has had the cold shivers on and off. She has been lethargic every time this has happened. Yesterday she slept all day (had 3 attacks and was quite shakey, glassy eyes and just generally unwell) but today she has had more energy until this afternoon when she got quite sleepy and had another attack. She was on Urinary SO food and we were in the process of changing it over to a new food during this time as well. She had quite a bad reaction to the food changeover (black runny poo) so we have since put her back on the urinary so food. Her lymph nodes behind her jaw are a little swollen. She has also been incessantly licking lips and other things(last night we woke up with wet sheets because she was licking them all night). We took her to the vet today but when we went she was feeling a lot better. So we decided to wait and see. Now it has happened again and she is lethargic again. Does anyone have any idea what might be going on?