I understand your concern for your girl Chanel's muscle spasms.
I appreciate the history of a recent heat and giving Trifexis. I don't think either are necessarily related as neither should cause muscle spasms.
It is certainly possible with your description of focal, rhythmic, uncontrollable spasms that she is having a petit mal seizure. These seizures affect a small group of muscles in a particular area of the body. They are not of the same significance as grand mal seizures (loss of consciousness, whole body, rhythmic, uncontrollable muscle contractions with possible loss of urine and stool continence) because the chance of body temperature increasing and subsequent brain damage is much less.
Given her breed another possible cause of her symptoms is intervertebral disc disease. This is when the spongy discs between the veretbrae in her spine either prolapse or leak and put pressure on her spinal cord. This is quite painful and can lead to painful muscle spasms or if there is enough pressure then even paralysis can result. Keeping her very quiet, no running, jumping or bending over (elevate her food and water bowls to head height) is best.
There are several other things that can cause muscle contractions other than petite mal seizures or disc disease. Infections, toxins (especially pyrethrin based flea products or organophosphate insecticides), nerve damage or inflammation, mineral (especially calcium) or electrolyte disturbances can all cause muscle fasiculations (spasms) as well.
I assume that she has not had any topical flea products applied as she is taking trifexis but I need to ask if you recently applied any topical flea medications or could she have been exposed to an insecticide? If so you want to bathe her thoroughly to remove as much of the toxin as possible with something like Dawn dishwashing soap and cool water. Do not use hot water or scrub her skin as that will increase toxin absorption.
If she seems to be progressing in symptoms and has a grand mal seizure (unconscious, unaware of surroundings, repetitive, uncontrollable whole body muscle movements with or without loss of urine or stool) then she should be seen by a veterinarian on an emergency basis tonight.
If her color is good and she is eating, drinking and behaving normally otherwise I suspect that she will do fine tonight and then tomorrow your veterinarian can examine her and run some tests to determine why she is having these muscle spasms. I wouldn't wait long to have her seen as some of the things that can cause muscle spasms (like toxins, electrolyte or mineral imbalances) can cause long term problems for your pup and may be indicative of serious diseases. If this is intervertebral disc disease it is very important to relieve the inflammation so it does not progress to paralysis.
Please let me know if you have any further questions.
Thanks for the further information. I am glad to hear that a toxin is less likely and that she did eat for you.
She may be very stoic and the only sign of her disc disease may be these muscle spasms, aggravated by movement. But given her previous history of seizures it is possible that she is having petit mal seizures.
I would definitely confine her, raise her food and water bowls to head height and have her veterinarian examine her as soon as possible. If you can videotape one of these episodes your veterinarian will be very appreciative as that can help in diagnosis.
You can also try putting a heat source (like a heating pad set on low/medium) on her neck near/on the area where these spasms are occuring. If these are muscle spasms secondary to disk disease the heat will help relax the muscles and she should feel much better.