It sounds like you might be having muscle spasms. This could cause intense pain and make the skin seem to flutter or move. I am not sure what you mean by it not being sciatica proper but I would not rule out the possibility of sciatica or at least something abnormal occurring in the nerve. There are also localized movements of muscle twitching (fibrillations) that could cause this; they could be due to some abnormality or condition developing either in the muscle fibers or nerve endings going into the muscles. If you are having other problems like weakness or numbness in that leg, that would also suggest a problem with the nerves and/or muscle. If this is getting worse, you might want to see a neurologist. Hope this helps.