very strange symptom

I have researched "symptoms" and found nothing on a very strange episode I experience about a week ago. It lasted only a second or two, but scared me. I was lying in bed trying to go to sleep and I felt what I can only describe as my brain shake....I heard and felt it. Right away my heart starting beating rapidly, then it was over. I fell asleep shortly after that and had not had an further feelings such as this.

Brief medical history: parathyroidectomy in Nov. 07, diagnosed with fibromyalgia in Jan 08, medications include Synthroid, Zocor, Prilosec, HCTZ, Enablex, Effexor and Hydrocodone (as needed for pain). All are generic.

I also have a herniated cervical disk.

Thanks, Diane