Realizing that you suddenly fit into your skinny jeans may seem like a pleasant surprise — but the reality is that unexplained weight loss (or weight gain, for that matter) is usually a red flag. If you've lost 10 pounds or more in less than a month (without changing your diet or exercise routine), you should see your doctor.
Unexplained weight loss can be an early sign of pancreatic, stomach, esophageal or lung cancer (as well as other health problems, like an overactive thyroid, diabetes, depression or liver disease).