Oh, goodness, Go! I can see why you are anxious. I'm not a doctor but what you have written doesn't describe symptoms of Alzheimer's that I can see. Go through that Mayo Clinic site I posted in my first post to you. This hospital is one of the very best in the US and I trust their information.
The pain in the arm.....that's exactly how my fibro hit me one night. I started getting pain in my feet and it worked up my body. When it got to my elbows, it was like a severely banged them. The "electrical-type" pain shot down both my arms and stayed that way for over two weeks. It's a very intense, horrible pain. Everyone with fibo has a different story as to how they came down with it.
I'm dealing with some light-headedness right now. This comes and goes for me. I live in Florida and the heat and humidity causes sinus problems and I think this is what is going on. It's probably hot and humid this time of year in South Africa, too.
Cognitive memory problems are very common with fibromyalgia. I've tried to start the car with the key to my freezer. I couldn't remember my phone number one time and had to get my checkbook out to see what it was! I have a very good mind but sometimes it stalls on me! 
This really kicked in when my husband died suddenly. I couldn't remember anything and my kids tell me we had conversations on subjects but I have absolutely no recollection of them! I immediately thought it was Alzheimer's too and that was frightening. I saw my doctor and he said it was fibromyalgia. That put my mind at ease and my memory improved. The stress and anxiety over this can cause your memory to be worse. I find if I just relax, laugh at myself and move on, I do just fine. I still have some problems but it doesn't prohibit me from my daily activities. Even the stress over your arm can kick in the "fibro fog". Here is a link about fibro fog that might help you.
http://webspace.webring.com/people/fc/cfsdays/follies.htm
Did your doctor run you through tests to rule out other illnesses? The symptoms of fibro are like other illnesses so they are ruled out before a diagnosis of fibro is given.
Be sure to check out Fibro 101...the second thread on the forum. There are good links to good information about fibro and you will learn a lot there. Good links to start with are called Symptoms, What Else Could It Be, and A Thorough Explanation of Fibromyalgia. Another link called Doctors Respond to New York Times Article is excellent too and might help your doctor if they believe that fibro is all in you head. It sure isn't!
Here are some teaching hospitals in South Africa. Perhaps one is near you and you might find a doctor that believes fibromyalgia is an illness. Here in the US, fibromyalgia is recognized by the American Medical Association, the American College of Rheumatology, the National Institute of Health and also the World Health Organization to name a few!
South Africa
By the way, could you do me a favor? Please use paragraphs when you write. It's really hard to read and I'm older and get lost and have to start over again! I separate my paragraphs with two spaces and that makes it easier to read, I think. Thanks in advance!
I hope this has helped you some. Please try to relax, think positive thoughts and enjoy each new day. You will get answers soon. Don't hesitate to ask more questions because we are her to help you. Feel better!
Sherrine