Its fairly common - it has to do with facial nerves and muscles dysfunctioning or being compressed or interferred with. Before my tmj started I noticed the left corner of my mouth pulled slightly downward occasionally and I felt like I was drooling a bit or extra saliva was seeping out of the corner of my mouth. Shortly thereafter I started to get an upper lip twitch just above the left corner near my upper canine. The involuntary movement is acutally a spasm, so if you are not seeing a specialist yet, you should b/c it can progress and become chronic.