Dear Ewe -
Feeling tired and weak are very common symptoms that can be caused by a wide variety of different sources, including anemia (low iron), MS, lupus, thyroid problems, and many other conditions. You need to see a doctor as soon as possible. Describe ALL new or recurring symptoms, any recent changes in your lifestyle or medications or supplements, and any other pertinent information. Emphasize that these are new symptoms for you. Your doctor should test you for a variety of conditions that can cause fatigue and weakness.
One possible cause of exhaustion and heaviness in the legs is Chronic Fatigue Syndrome (CFS), an immune system disorder. However, with CFS there are normally other symptoms besides fatigue (despite its name). These symptoms may include recurring sore throats or swollen glands, muscle or joint pain, headaches, flu-like aches and/or feverish feeling, sleep problems, and/or exercise intolerance (sudden, severe worsening of symptoms within hours or days after exercise or other exertion). In fact, exercise intolerance is a unique, defining characteristic of CFS.
If you have some of these symptoms and think that CFS is a possibility, you might want to try the Do I Have CFS? self-assessment at the CFIDS Association website (CFIDS is another name for CFS). Here's the link:
There is no test (yet) to confirm CFS, but if your symptoms fit the CFS definition and all of your other tests are negative, then your doctor can diagnose you based on your symptoms. Here's a list of tests that should be done to exclude other conditions, provided by the CFIDS Association:
Fatigue is such a common and varied symptom of so many conditions that it can be difficult to find the cause. Don't give up! If your doctor isn't willing to work with you to get to the bottom of your symptoms, then find another. Something has changed for you, and there is a medical explanation, but it may take a lot of persistence to find it so that you can start treating the underlying cause.
Good luck. If you have other questions about CFS in particular, please feel free to ask me, either through this site or by visiting my blog at www.livewithcfs.blogspot.com.