Macrodantin (nitrofurantoin) is a broad spectrum antibiotic.
The usual dosage for dogs is 4.4 mg/kg, (2mgs per pound of body weight) orally 3 times daily, for 4–10 days.
Adverse effects are not common at normal doses, but nausea, vomiting, and diarrhea can be seen.
Rarely we can see hypersensitivity reactions with this drug, which could show up as hives, and red, itchy skin.
If your pup is 20 pounds then she would normally take 40mgs. If she ingested 50mgs that is not too far outside of a normal dose and I would not expect any long term effects from this one time dose, if she is otherwise healthy.
She may be somewhat nauseous and may have some loose stools, but that should not last.
If she seems nauseous I recommend feeding a bland diet of 1/3 boiled, shredded white chicken and 2/3 boiled white rice. You can mix in low salt chicken broth or warm water to make the food more palatable and get in more fluids.
If she starts vomiting she should see a veterinarian on an emergency basis for an injection to ease her nausea, intravenous fluids and supportive care.
Please let me know if you have any further questions.