How long has this been going on?
Any change in her diet? (Are you feeding the same as the breeder)
Any change in her stools?
Are vaccinations up to date?
When you say she is lethargic how lethargic is she?
Thank you for the additional information, anytime your dog is lethargic it is highly recommend to have her examined by her vet. Lethargic behavior is a very serious issue, due to the fact that a dog can be very very ill and still have energy. When a dog is lethargic that is a indication that the dog is very sick.
The parasites is the likely cause of this, but the problem is the wormer should not be making her this sick. I suspect that she is having a reaction to the wormer which can be very dangerous.
My recommendation is if she is lethargic to see her vet or get her to a emergency vet tonight.
If she is getting up and playing some that is good......she may simply be tired. But if she is unwilling to get up and go to the bathroom, eat, or play then it is time for a vet examination.
Different wormer's can cause different reactions, sometimes delayed reactions. It may also be something else that is causing the problem. I am concerned that the breeder did the vaccinations....sadly some breeders are dishonest on this and claim that vaccinations have been given when they have not.
It could also be a allergic reaction to something else, though i would bank on it either being a reaction to the wormer or something more serious like Parvo or Distemper.