Dear Valerie -
Sorry for the delay in responding to you - I was away on an extended vacation. As Alyson suggested above, I would suggest using the Good Doctor List to try to find a doctor who will take you seriously and work with you to get to the bottom of your illness.
It is certainly possible that you have Chronic Fatigue Syndrome or CFS (I have had it for 7 years). It can often start very abruptly - mine began on March 2, 2002 - and is often initially triggered by an infection of some sort, but the CFS persists after the infection has been treated.
However, some doctors use CFS (or the more generic term chronic fatigue) as a catch-all term when they don't know what's really wrong (or they don't believe there's a real medical problem). This is unfortunate because CFS is a very real illness with defined symptoms, but it is the reality with some doctors who haven't kept up to date with the latest research. Many, many different conditions can cause fatigue and muscle weakness.
The best source of information on CFS is the CFIDS (another word for CFS) Association. Their website is very informative, accurate, and up-to-date and even includes a self-quiz to help tell if you have CFS:
Here's their page with an overview of how to diagnose and treat CFS:
If you think your symptoms match what you read there, then you should definitely try to find a doctor who truly understands CFS and knows how to treat it because there are many ways to help alleviate the symptoms, even though there's no cure (yet!). The Good Doctor List is a good place to start. One of the key, unique characteristics of CFS is something called post-exertional malaise (an awful euphemism!). It means that any sort of physical or mental exertion can cause a sudden increase in symptoms (a "crash") afterward. So, maybe you feel good one day and take a walk, but within a few hours - or maybe the next day - you suddenly feel as if you have a terrible flu. Also, CFS is much more than just fatigue - it always includes flu-like symptoms which might include feeling feverish (whether or not your temp is actually high); vague, flu-like aches; recurring sore throat or swollen glands; etc.
That About CFIDS page that I listed above also includes a lengthy list of other diagnoses that should be excluded (on the diagnose page) - this is a good starting point for looking for other possible answers, if your symptoms don't seem to fit CFS. Even if you do have CFS, it is often triggered by an infection, so you want to be sure any underlying infections are treated (and, again, a knowledgable CFS doctor can help with that).
Most importantly, don't give up. I know how frustrating your situation is because most of us with CFS had trouble getting an accurate diagnosis. It took me a year of serious illness before I was correctly diagnosed, and I'm one of the lucky ones!
Quit seeing any doctor who thinks you're making it up or that it's psychosomatic - you know something changed and there must be a medical explanation for it. And of course you're anxious and depressed! Anyone with a mysterious and debilitating illness with no clear diagnosis would be. Make sure any doctor you see understands that depression is a common EFFECT of any chronic illness and that your physical symptoms began first (don't even mention depression or anxiety to a new doctor). When you see a new doctor, go in with carefully documented facts - exactly when symptoms began, any previous test results, progression of symptoms or new symptoms, etc. I know it's discouraging, but there must be a doctor out there who can help you find the answers you need.
Good luck -