I would be inclined to think it is muscle and positional pain. Can you think back at what you may have been doing when this pain and cramping appear? Ally
The underlying cause can be neuroligical or muscle. But returning once a month is puzzling...do you have changes in sleeping positions, activities that could be related once a month...it does't appear to be anyheart related problem.
Thanks for the question, and if you have any further questions or comments you are welcome to respond. Take care,
Ken