Suggest Remedy For Sudden Weight Loss

Yes. My wife weighed 136 pound a few months ago. For some reason she lost 10 pounds within the last 6 weeks. She has been to the doctor and had tests done but she was told that they did not detect a reason why. The doctor advised her to get rid of stress if she had any. Since the past couple of weeks she has been eating well trying to gain weight back. She has gained a pound or two but within a few days she has lost back to 126. Might there be a problem with her metabolism?