Hi there.
I am so sorry you are going through this. One never thinks that something as routine as a spay surgery could cause so much heartache.
This is very rare, I definitely agree with that. I would be concerned of how sterile the environment in which your dog was spayed was, how sterile the surgical instruments were, ect. You may want to look into this.
As a course of treatment, your dog should receive tetanus anti-toxin, fluids and antibiotics. The anti-toxin is used to neutralize the effects of the toxin which was produced as the bacteria grew. Antibiotic kills the Clostridium tetani bacteria, and the fluids are needed to replenish the dog's system, since she probably has had difficulty eating and drinking. Left untreated, canine tetanus can result in cardiac problems, seizures and death. Esophageal hiatal hernias and enlargement and thickening of the esophagus have also been associated with canine tetanus.
Tetanus is an acute, noncontagious, neurotoxic disease of domestic and other animals and humans. It is characterized by painful tonic and clonic spasms of skeletal muscles. Tetanus is rare in dogs and cats; all ages are susceptible.
Breaking down this ailment may help give you more information:
Etiology - Clostridium tetani, a large, gram-positive, sporeforming, anaerobic rod. The highly resistant spores are present in the soil and feces of animals.
Distribution - The disease is worldwide in distribution.
Mode of Infection - The spores of C. tetani enter a variety of wounds and if necrosis is present they may germinate and produce neurotoxin in the anaerobic milieu. Wounds are not always found.
Clinical Features - There is a gradual onset of stiffness, with hyperesthesia developing, followed by tetanic spasms of all voluntary muscles.
Diagnosis - This disease is so characteristic clinically that a laboratory diagnosis is not required.
If it is desired to attempt isolation and identification of C. tetani, material (pus, necrosed tissue) from the wound should be taken so as to minimize exposure to air, preferably in an anaerobic transport system.
Gram-stained smears from a wound may reveal the characteristic drumstick (terminal spore) bacteria; however, their absence does not exclude tetanus.

Treatment - Expose, clean and disinfect wound.
Muscle relaxants and sedatives.
Penicillin and an adequate dose of tetanus antitoxin; intrathecal administration of antitoxin is used by some clinicians. If the wound is found penicillin can be inoculated into the area around it.
Water and food may have to be administered.
Some dogs and cats will recover with treatment prolonged supportive care.

Control - To prevent tetanus in a wounded animal, clean and disinfect the wound.
Because the disease is rare in dogs and cats immunization is not ordinarily recommended.

It may be beneficial to have your veterinarian debride the spay incision and flush it out with sterile saline to flush out the bacteria and infection. An injectible antibiotic like Cefazolin should be administered to the cavity of the spay incision.
I hope this information has been helpful. If you have any other questions, please feel free to ask!