This doesn't seem to be a case of post exertional malaise (PEM) because I didn't do anything out of the ordinary yesterday. I suspect there may be a dietary cause because I consumed much more dairy than ordinary yesterday, and I have also been eating strips of prociutto as part of my dinner. Several authors in the book "Recovery from CFS" mentioned the elimination of pork from their diets as a key factor in CFS diets. I couldn't understand why pork would be different from any other meat until I read the following tonight, on a website concerning candida diets:
I'll have to experiment further and see if dairy or pork is a possible culprit.