I went back to see the doctor yesterday. The results of the blood tests I had almost a month ago showed nothing obviously wrong (the white blood cell count, which was low in my first test, had picked up by the second a few days later) so diagnosis is very difficult. The symptoms have remained pretty much the same, though if anything the have grown a little worse. Some days are better than others. Today, for example has been pretty bad, so far.
Anyway, the doctor examined me but couldn't find anything unusual - my blood pressure was normal and my chest sounded fine. At the moment, her guess is that I am recovering from some sort of viral infection (that I may not have been aware of). She said that it can sometimes take a couple of months to get back up to strength. Personally, I don't subscribe to this theory as it has been going on for quite a lot longer than that. However, I only went to the doctor's for the first time a month ago so for know I'll go along with it. She also said that it could be stress related or maybe even a vitamin deficiency that the tests don't show up. Assuming the condition doesn't deteriorate too rapidly then I am prepared to see how it goes for a while.
Just to be absolutely sure, I went for another round of tests this morning, repeating some I've already had and adding a test for thyroid function, just to rule that out as well. I don't expect them to show anything but we'll see.
However, I have also decided to become more pro-active about the whole thing. First of all, I have bought (at the doctor's suggestion) some vitamin supplements, just in case that is the problem - I doubt it is but you never know for sure. The doctor also said that getting some exercise may help. As perverse as the idea of exercising when you don't feel like you've got enough energy to walk up the stairs sounds, I'll give it a go. Lastly, and this idea is completely my own, I'm going to keep some sort of diary of symptoms to try and get a handle on when they affect me. That way I might be able to work out why.
Of course, time will tell whether any of this will make any difference. I don't think it will but it's worth doing just to eliminate some of the possible causes. In a few more weeks, if I'm not feeling better I'll go back and see what else it could be. Maybe I'll even look towards the alternative side of medicine. That'd be a turn up for an old cynic like me.