I get these all the time. Involuntary muscle spasms. My bf laughs at me when it happens. I've just put it down to the fact that as our bodies and muscles are relaxing and due to the build up of tension during the day, they tend to twitch as they relax.

Now this is only an uneducated guess and assumption. I'm not sure if this is the reason. I wouldn't worry though as I highly doubt it is a medical problem. As long as you aren't hurting yourself in the process, just accept it and laugh at it. I certainly do :p