What causes sudden dizziness and nausea with balance problems?

Vertigo. The symptoms you describe suggest a diagnosis of an inner ear problem called labarynthitis. Causes are multiple, some benign and some serious. If symptoms are persistant, serious problems are more likely. See your family doctor to discuss this matter further and to determine if further evaluation is required.
Vertigo. Benign vertigo is the most common cause of sudden dizziness associated with nausea and gait imbalance. Other common causes include dehydration, infection.
Vertigo. This may be a cause of vertigo. Unfortunately, often times it is very difficult to find the cause of it when this occurs the best thing to do is keep your eyes fixed on an object until the feeling goes away. You should be evaluated to rule out any reversible causes.