Frightening Dizziness

I often experience extreme lightheadedness, dizziness, loss of peripheral vision, feeling faint, and muscle spasms/sudden loss of strength in my arms or legs when I stand up.  I have a desk job, and have always attributed these episodes to standing up too fast.  However, these episodes are becoming alarmingly frequent and intense, and I often have to lean against a wall to avoid falling down, for up to five minutes.  I am overweight, but my blood pressure is normal.  I take 20mg. flouexitine daily, at bedtime.  I am worried that this is more serious than inner ear or low blood pressure.  Any thoughts?

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Orthostatic hypotension symptoms include:

Feeling dizzy or lightheaded upon standing
Syncope (fainting)
Blurred vision
Nausea
Disorientation or confusion
Feeling weak
Fatigue
Falling
Chest pain

Syndromes with orthostatic dizziness or lightheadedness, not associated with low blood pressure include:

* Positional orthostatic tachycardia (POT) syndrome. The pulse races on standing.
* Low CSF pressure syndrome
* Primary orthostatic tremor
* Positional vertigo (i.e. BPPV)

Other causes of orthostatic hypotension include the following:

* Aging
* Cardiogenic (heart related) orthostatic hypotension.
* Hypovolemia (a drop in the volume of blood).
* Dehydration (low fluid volume in the body)
* Immobility (for instance, staying in bed for a long time)
* Primary adrenal insufficiency
* Pregnancy
* Heart conditions, including heart attack, heart failure, and valve disease
* Anaemia (low red blood cell count)
* Neurogenic orthostatic hypotension
* Sensory neuropathies
* Diseases of the endocrine system, including diabetes and thyroid conditions
* Medication (high doses of blood pressure medications or medications for depression)
* Hot weather
* Multiple system atrophy, Shy-Drager, Parkinson's, dementia with Lewy bodies.  
* Medullary strokes or injuries (rare)
* Wernickes syndrome (rare, related to thiamine deficiency)
* Peripheral neuropathy, especially diabetes and amyloidosis
* Spinal cord lesions
* PAF - pure autonomic failure or idiopathic orthostatic hypotension.
* Dopamine beta-hydroxylase deficiency (hereditary, very rare)

http://www.dizziness-and-balance.com/disorders/medical/orthostatic.html
http://my.clevelandclinic.org/disorders/orthostatic_hypotension/hic_orthostatic_hypotension.aspx