Lupus can cause several different types of nerve damage. Lupus can also cause several different types of muscle damage. Some medications can cause muscle spasm. I am sorry, but there are so many possibilities for me to give you any indication of what is causing this problem. I know that you are afraid and I am not offering you much help or comfort. I would suggest that you contact your doctor as soon as you can, explain the symptom and ask to bee seen for evaluation.
Please let us know what you find out!
Peace and Blessings