A 30 year old woman with depression presents with dizziness after being on a boat (or using a flight simulator or being on a water bed). When she's on a boat, she feels a sense of swaying or rocking, like she's walking on uneven ground. She doesn't feel like the room is spinning around her, but rather she just has disequilibrium. She does not feel nauseous. She's never had motion sickness or seasickness. Her physical examination is normal. Vestibular tests are normal. She is treated with benzodiazepines.
Challenge: What's the diagnosis?