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In all probability the symptoms are due to panic attack. Other than that tremors are often associated with drugs, alcohol, smoking, lack of sleep, poor sleep patterns, stress and fatigue. They can be seen in peripheral neuropathies of diabetes, in hyperthyroidism, parasomnia, multiple sclerosis, TIAs or transient ischemic attacks or mini strokes, hypoglycemia, disorders of calcium metabolism, hyperparathyroidism etc. Certain epilepsies too could be the cause, probably ruled out in your case. Consult an internal medicine specialist or your PCP. Discuss these possibilities with your doctor. A comprehensive investigation is required keeping all the points in mind.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!