My name is***** and I am sorry to hear that Zoey is not feeling well this evening. Based on your history, I would wonder if she is in pain and if she is not moving her head, my first thought would be neck pain.
Neck pain is common in Chihuahuas because of blows disks in their backs.
Here is a link to a discussion on disk disease in dogs:
Other causes of pain include stomach or abdominal pain or lower back pain. I highly doubt that the flea medication is the problem.
For tonight, I would keep her as comfortable as possible and don't ask her to move around. If she seems very painful, you may need to take her to your local animal emergency room.
If she is able to get comfortable in her bed, then you can wait until morning to take her to your regular vet for an exam.
Best regards, ***** ***** this answer helps you.
Feel free to contact me if you have any more questions.
Kansas State University,
College of Veterinary Medicine Class of 2001