Hi Ocean, Lupus is known as the disease of a thousand faces because it can present so differently in so many people which makes it very hard to diagnose by symptoms, so the best bet is to get the tests done and see what results you get.
Dermatomyositis is also an auto immune disease, but with all diseases the symptoms can be very different from person to person, so you dont necessarily always get a perfect match in symptoms, so again testing is the only way to be sure. The blood tests for it are creatine phosphokinase and aldolase, which show if there is muscle damage and should be done for you if you have significant muscle wastage, the other blood tests are for auto anti bodies like ANA these tests are all explained here http://emedicine.medscape.com/article/332783-workup#aw2aab6b5b2
I get the feeling from what you have been saying that you havent been very extensively worked up by doctors and there are a lot of tests that havent been done, and symptoms that havent been investigated properly, you need to get a doctor to properly investigate all the possible causes of your symptoms, to make sure that nothing has been missed, as you say yourself you have a lot of symptoms that dont really fit with a ME diagnosis, so it is important to make sure that every other possibility is ruled out. A lot of people are making out that having PEM means you have ME, this is not true it is found in many conditions.
Regarding Lyme it is a very controversial subject, but the truth is that none of the tests currently available are proven to be 100% accurate. Which leads to people getting false positives and then having wrong and dangerous treatment for a disease they havent got, while their true diagnosis gets missed, so be cautious, Unfortunately a very accurate test has been developed called VOVO LIPS but at present it isnt widely available largely due to financial reasons see http://lymediseaseguide.org/lyme-disease-test-the-promise-of-vovo-lips
My view is that you need to make sure that you have been tested for everything that could possibly cause your symptoms, and make sure that nothing has been missed, yes Lyme is possible but so are a lot of other things, so make sure they are all ruled out no matter what any Lyme test says, if you get a positive lyme test, it could be a false positive, and if you focus on trying to treat that before everything else has been ruled out something important could be missed. So be cautious and get more testing done before accepting any diagnosis.
I think the fact that you got so much better after taking prednisone could be very important in finding the correct diagnosis, its a very strong drug and the diseases that it treats are well known. They are diseases that cause low Cortisol, adrenal and pituitary disorders like Addisons disease, have your cortisol levels been tested?? And diseases like Lupus, MS, allergic reactions, Sarcoidosis etc, etc, So in theory if a doctor gets a list of all the diseases that prednisone is used to treat, and then compares your symptoms to those on the list of diseases that prednisone is used for, they should be able to narrow it down to a few, and then test for them and hopefully come up with the right diagnosis.
All the best
Dermatomyositis is also an auto immune disease, but with all diseases the symptoms can be very different from person to person, so you dont necessarily always get a perfect match in symptoms, so again testing is the only way to be sure. The blood tests for it are creatine phosphokinase and aldolase, which show if there is muscle damage and should be done for you if you have significant muscle wastage, the other blood tests are for auto anti bodies like ANA these tests are all explained here http://emedicine.medscape.com/article/332783-workup#aw2aab6b5b2
I get the feeling from what you have been saying that you havent been very extensively worked up by doctors and there are a lot of tests that havent been done, and symptoms that havent been investigated properly, you need to get a doctor to properly investigate all the possible causes of your symptoms, to make sure that nothing has been missed, as you say yourself you have a lot of symptoms that dont really fit with a ME diagnosis, so it is important to make sure that every other possibility is ruled out. A lot of people are making out that having PEM means you have ME, this is not true it is found in many conditions.
Regarding Lyme it is a very controversial subject, but the truth is that none of the tests currently available are proven to be 100% accurate. Which leads to people getting false positives and then having wrong and dangerous treatment for a disease they havent got, while their true diagnosis gets missed, so be cautious, Unfortunately a very accurate test has been developed called VOVO LIPS but at present it isnt widely available largely due to financial reasons see http://lymediseaseguide.org/lyme-disease-test-the-promise-of-vovo-lips
My view is that you need to make sure that you have been tested for everything that could possibly cause your symptoms, and make sure that nothing has been missed, yes Lyme is possible but so are a lot of other things, so make sure they are all ruled out no matter what any Lyme test says, if you get a positive lyme test, it could be a false positive, and if you focus on trying to treat that before everything else has been ruled out something important could be missed. So be cautious and get more testing done before accepting any diagnosis.
I think the fact that you got so much better after taking prednisone could be very important in finding the correct diagnosis, its a very strong drug and the diseases that it treats are well known. They are diseases that cause low Cortisol, adrenal and pituitary disorders like Addisons disease, have your cortisol levels been tested?? And diseases like Lupus, MS, allergic reactions, Sarcoidosis etc, etc, So in theory if a doctor gets a list of all the diseases that prednisone is used to treat, and then compares your symptoms to those on the list of diseases that prednisone is used for, they should be able to narrow it down to a few, and then test for them and hopefully come up with the right diagnosis.
All the best