Dog suddenly starts trembling?

My 6 year old Maltese (healthy weight) was lying on the couch next to me when there was a very sudden onset of trembling. She tried to stand up but seemed disoriented, her legs were tensed and it was like she couldn't stand. No drooling, blinking, or disruption in breathing. This spasm lasted for less than 30 seconds. Afterwards she went entirely back to normal. Any ideas what was wrong?