Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand your concern for your girl because you are noticing her having rear leg muscle spasms.
It is certainly possible with your description of muscle spasms of only a particular group of muscles that your girl is having petit mal seizures. 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.
There are several things that can cause muscle contractions other than petite mal seizures. Infections, nerve damage or inflammation, toxin exposure, kidney disease and mineral (especially calcium) or electrolyte disturbances can all cause muscle fasciculations (spasms) as well.
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. Some flea control products, especially the over the counter products with high amounts of pyrethrin type insecticides can cause muscle twitches in sensitive dogs.
Another possible cause of her symptoms is intervertebral disc disease. This is when the spongy discs between the vertebrae 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.
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 today.
If her color is good and she is eating, drinking and behaving normally otherwise I suspect that she is stable but I do recommend that your veterinarian examine her and run some tests to determine why she is having these muscle spasms. I wouldn't wait too 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 girl 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. If this is secondary to high blood calcium levels then addressing that promptly so there is no organ damage and tissue calcification is best.
Please let me know if you have any further questions.