I had these as well. I actually started getting them as my other anxieties started winding down. It seemed as those as soon as I got use to one of the physical symptoms of anxiety another one would appear.

I still get these jerks occasionally. I don't know anything about your diet, but I noticed mine were worse when I had too many stimulants durning the day - sugar, caffeine, nicotine etc. Or if I am really tired. Now I avoid these almost entirely and the jerks really only kick up on bad days. Now, when I do have one I can usually point to something I ate or drank that day as the culprit. I also take Xanax occasionally before bed if I've been in a "high anxiety" mode.